U.S. patent number 5,955,140 [Application Number 08/746,680] was granted by the patent office on 1999-09-21 for low volatility solvent-based method for forming thin film nanoporous aerogels on semiconductor substrates.
This patent grant is currently assigned to Texas Instruments Incorporated. Invention is credited to William C. Ackerman, Bruce E. Gnade, Shin-Puu Jeng, Gregory P. Johnston, Alok Maskara, Teresa Ramos, Douglas M. Smith, Richard A. Stoltz.
United States Patent |
5,955,140 |
Smith , et al. |
September 21, 1999 |
Low volatility solvent-based method for forming thin film
nanoporous aerogels on semiconductor substrates
Abstract
This invention has enabled a new, simple thin film nanoporous
dielectric fabrication method. In general, this invention uses
glycerol, or another low volatility compound, as a solvent. This
new method allows thin film aerogels/low density xerogels to be
made without supercritical drying, freeze drying, or a surface
modification step before drying. Thus, this invention allows
production of nanoporous dielectrics at room temperature and
atmospheric pressure, without a separate surface modification step.
Although this new method allows fabrication of aerogels without
substantial pore collapse during drying, there may be some
permanent shrinkage during aging and/or drying. This invention
allows controlled porosity thin film nanoporous aerogels to be
deposited, gelled, aged, and dried without atmospheric controls. In
another aspect, this invention allows controlled porosity thin film
nanoporous aerogels to be deposited, gelled, rapidly aged at an
elevated temperature, and dried with only passive atmospheric
controls, such as limiting the volume of the aging chamber.
Inventors: |
Smith; Douglas M. (Albuquerque,
NM), Johnston; Gregory P. (Albuquerque, NM), Ackerman;
William C. (Albuquerque, NM), Stoltz; Richard A. (Plano,
TX), Maskara; Alok (Albuquerque, NM), Ramos; Teresa
(Albuquerque, NM), Jeng; Shin-Puu (Plano, TX), Gnade;
Bruce E. (Dallas, TX) |
Assignee: |
Texas Instruments Incorporated
(Dallas, TX)
|
Family
ID: |
27585440 |
Appl.
No.: |
08/746,680 |
Filed: |
November 14, 1996 |
Current U.S.
Class: |
438/778; 427/100;
427/126.3; 427/422; 427/427; 257/E21.273; 427/96.7; 438/783 |
Current CPC
Class: |
H01L
21/02337 (20130101); H01L 21/02343 (20130101); H01L
21/02126 (20130101); H01L 21/31695 (20130101); H01L
21/02203 (20130101); H01L 21/02216 (20130101); H01L
21/02282 (20130101); H01L 21/76801 (20130101) |
Current International
Class: |
H01L
21/316 (20060101); H01L 21/02 (20060101); H01L
21/314 (20060101); B05D 005/12 (); B05D
001/02 () |
Field of
Search: |
;427/421,422,427,126.3,100,96 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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382310 |
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Aug 1990 |
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EP |
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0 382 310 A2 |
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Aug 1990 |
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EP |
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454239 |
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Oct 1991 |
|
EP |
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0 454 239 A2 |
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Oct 1991 |
|
EP |
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WO 92/03378 |
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Mar 1992 |
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WO |
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Other References
VS. Klimenko, L.A. Kulik, and V.V. Vashchinskaya, Dependence of the
Composition and Structure of Silicic Acid Xerogels on the Nature of
the Solvent, 1986, Ukrainskii Khimicheskii Zhurnal, vol. 52, No.
12, pp. 1247-1251. .
Norges Tekniske Hogskole, Preparation and Characterization of
Transparent, Monolithic Silica Xerogels with Low Density, Jan.
1993. Cursory Consideration. .
D. Basmadjian, G. N. Fulford, B.I. Parsons, and D.S. Montgomery,
The Control of the Pore, Volume and Pore Size Distribution in
Alumina and Silica Gels by the Addition of Water Soluble Organic
Polymers Dec. 1962, Journal of Catalysis, vol. 1, No. 6, pp.
547-563. Cursory Consideration. .
Siv Haereid, Preparation and Characterization of Transparent,
Monolithic Silica Xerogels with Low Density, Jan. 1993, Norges
Tekniske Hogskole Universiteteti I Trondheim. .
H. Yokogawa, M. Yokoyama, Hydrophobic Silica Aerogels, Journal of
Non-Crystalline Solids 186 (1995) 23-29..
|
Primary Examiner: Lusignan; Michael
Assistant Examiner: Chen; Bret
Attorney, Agent or Firm: Denker; David Holland; Robby T.
Donaldson; Richard L.
Parent Case Text
This application claims the benefit of priority from the following
U.S. provisional applications:
Filing Attorney Date Appl.# Docket Title
Nov. 16, 1995 60/006,852 TI-21620P Rapid Aging Technique for
Aerogel Thin Films.
Jan. 24, 1996 60/010,511 TI-21622P Nanoporous Dielectric Thin Film
Surface Modification
Nov. 16, 1995 60/006,853 TI-21623P Aerogel Thin Film Formation From
Multi-Solvent Systems.
Nov. 16, 1995 60/006,861 TI-21624P Nanoporous Dielectric Formation
Using A Post-Deposition Catalyst.
Mar. 4, 1996 60/012,764 TI-22177P Glycol-Based Method for Forming a
Thin Film Nanoporous Dielectric.
Mar. 4, 1996 60/012,765 TI-22778P Glycol-Based Precursors For
Aerogels
Mar. 4, 1996 60/012,763 TI-22779P Glycol-Based Method For Forming a
Thin Film Aerogel on a Passive Substrate
Mar. 4, 1996 60/012,799 TI-22780P Glycol-Based Method For Forming
Bulk Aerogels
Mar. 25, 1996 60/014,009 TI-22781P Polyol-Based Precursors For
Aerogels
Mar. 25, 1996 60/014,005 TI-22782P Polyol-Based Method For Forming
Thin Film Aerogels On Semiconductor Substrates
Mar. 25, 1996 60/014,008 TI-22783P Polyol-Based Precursors For
Forming a Thin Film Aerogel on a Passive Substrate
Mar. 25, 1996. 60/014,146 TI-22784P Polyol-Based Method For Forming
Bulk Aerogels
Mar. 4, 1996. 60/012,800 TI-22788P Low Volatility Solvent-Based
Method For Forming Aerogels
Jul. 31, 1996. 60/022,842 TI-23260P Devices to Heat Treat Saturated
Porous Films
Claims
What is claimed is:
1. A method for forming a thin film nanoporous dielectric on a
semiconductor substrate, the method comprising the steps of:
a) providing a semiconductor substrate comprising a microelectronic
circuit;
b) depositing an aerogel precursor sol upon the substrate; wherein
the aerogel precursor sol comprises
an aerogel precursor reactant selected from the group consisting of
metal alkoxides, at least partially hydrolyzed metal alkoxides,
particulate metal oxides, and combinations thereof, and a first
solvent comprising glycerol; wherein
the molar ratio of the molecules of the glycerol to the metal atoms
in the reactant is at least 1:16;
c) allowing the deposited sol to create a gel, wherein the gel
comprises a porous solid and a pore fluid; and
d) forming a dry, nanoporous dielectric by removing the pore fluid
without substantially collapsing the porous solid; wherein
the forming step is performed in a drying atmosphere, and the
pressure of the drying atmosphere during the forming step is less
than the critical pressure of the pore fluid.
2. The method of claim 1, wherein the molar ratio of the molecules
of the glycerol to the metal atoms in the reactant is no greater
than 12:1.
3. The method of claim 1, wherein the molar ratio of the molecules
of the glycerol to the metal atoms in the reactant is between 1:2
and 12:1.
4. The method of claim 1, wherein the molar ratio of the molecules
of the glycerol to the metal atoms in the reactant is between 1:4
and 4:1.
5. The method of claim 1, wherein the molar ratio of the molecules
of the glycerol to the metal atoms in the reactant is between 2.5:1
and 12:1.
6. The method of claim l, wherein the nanoporous dielectric has a
porosity greater than 60% and an average pore diameter less than 20
nm.
7. The method of claim 1 wherein the nanoporous dielectric has a
dielectric constant less than 2.0.
8. The method of claim 1, wherein the nanoporous dielectric has a
dielectric constant less than 1.8.
9. The method of claim 1, wherein the nanoporous dielectric has a
dielectric constant less than 1.4.
10. The method of claim 1, wherein the temperature of the substrate
during the forming step is above the freezing temperature of the
pore fluid.
11. The method of claim 1, wherein the temperature of the substrate
during the forming step is above the freezing temperature of the
pore fluid, and
wherein the method does not comprise the step of adding a surface
modification agent before the forming step.
12. The method of claim 1, wherein the temperature of the substrate
during the forming step is above the freezing temperature of the
pore fluid, and
the nanoporous dielectric has a porosity greater than 60% and an
average pore diameter less than 20 nm;
wherein the method does not comprise the step of adding a surface
modification agent before the forming step.
13. The method of claim 1, further comprising the step of aging the
gel.
14. The method of claim 13, wherein at least part of the aging step
is performed in a substantially closed container.
15. The method of claim 13, wherein the temperature of the gel
during the aging is greater than 30 degrees C.
16. The method of claim 13, wherein the temperature of the gel
during the aging is greater than 80 degrees C.
17. The method of claim 13, wherein the temperature of the gel
during the aging is greater than 130 degrees C.
18. The method of claim 1, wherein the porous solid has less than
2% permanent volume reduction during the pore fluid removal.
19. The method of claim 1, wherein the porous solid remains
substantially uncollapsed after the pore fluid removal.
20. The method of claim 1, wherein the porous solid has less than
5% volume reduction during the pore fluid removal.
21. The method of claim 1, wherein the porous solid has less than
1% volume reduction during the pore fluid removal.
22. The method of claim 1, wherein the allowing step is performed
in a gelation atmosphere, wherein the concentration of a vapor of
the first solvent in the gelation atmosphere is not actively
controlled.
23. The method of claim 1, wherein the allowing step is performed
in a gelation atmosphere, wherein the concentration of a vapor of
the first solvent in the gelation atmosphere is substantially
uncontrolled.
24. The method of claim 1, wherein the reactant is a metal alkoxide
selected from the group consisting of tetraethoxysilane,
tetramethoxysilane, methyl-triethoxysilane,
1,2-Bis(trimethoxysilyl)ethane and combinations thereof.
25. The method of claim 1, wherein the reactant is
tetraethoxysilane.
26. The method of claim 1, wherein the dry, porous dielectric has a
porosity greater than 60%.
27. The method of claim 1, wherein the dry, porous dielectric has a
porosity between 60% and 90%.
28. The method of claim 1, wherein the dry, porous dielectric has a
porosity greater than 80%.
29. The method of claim 1, further comprising the step of replacing
at least part of the pore fluid with a liquid before the removing
pore fluid step.
30. The method of claim 29, wherein the liquid comprises
hexanol.
31. The method of claim 1, further comprising the step of annealing
the dry, porous dielectric.
32. The method of claim 1, wherein the pressure of the drying
atmosphere during the forming step is less than 3 MPa.
33. A method for forming a thin film nanoporous dielectric on a
semiconductor substrate, the method comprising the steps of:
a) providing a semiconductor substrate comprising a microelectronic
circuit;
b) depositing an aerogel precursor sol upon the substrate; wherein
the aerogel precursor sol comprises
a metal-based aerogel precursor reactant,
a first solvent comprising glycerol, and
a second solvent; wherein
the molar ratio of the molecules of the glycerol to the metal atoms
in the reactant is at least 1:16;
c) allowing the deposited sol to create a gel, wherein the gel
comprises a porous solid and a pore fluid; and
d) forming a dry, nanoporous dielectric by removing the pore fluid
in a drying atmosphere, wherein
the pressure of the drying atmosphere during the forming step is
less than the critical pressure of the pore fluid.
Description
FIELD OF THE INVENTION
This invention pertains generally to precursors and deposition
methods for thin film nanoporous aerogels on semiconductor
substrates, including deposition methods suited to aerogel thin
film fabrication of nanoporous dielectrics.
BACKGROUND OF THE INVENTION
Aerogels are porous silica materials which can be used for a
variety of purposes including as films (e.g. as electrical
insulators on semiconductor devices or as optical coatings) or in
bulk (e.g. as thermal insulators). For ease of discussion, the
examples herein will be mainly of usage as electrical insulators on
semiconductor devices.
Semiconductors are widely used in integrated circuits for
electronic devices such as computers and televisions. Semiconductor
and electronics manufacturers, as well as end users, desire
integrated circuits which can accomplish more in less time in a
smaller package while consuming less power. However, many of these
desires are in opposition to each other. For instance, simply
shrinking the feature size on a given circuit from 0.5 microns to
0.25 microns can increase energy use and heat generation by 30%.
Miniaturization also generally results in increased capacitive
coupling, or crosstalk, between conductors which carry signals
across the chip. This effect both limits achievable speed and
degrades the noise margin used to insure proper device operation.
One way to reduce energy use/heat generation and crosstalk effects
is to decrease the dielectric constant of the insulator, or
dielectric, which separates conductors. U.S. Pat. No. 5,470,802,
issued to Gnade et al., provides background on several of these
schemes.
A class of materials, nanoporous dielectrics, includes some of the
most promising new materials for semiconductor fabrication. These
dielectric materials contain a solid structure, for example of
silica, which is permeated with an interconnected network of pores
having diameters typically on the order of a few nanometers. These
materials may be formed with extremely high porosities, with
corresponding dielectric constants typically less than half the
dielectric constant of dense silica. And yet despite their high
porosity, it has been found that nanoporous dielectrics may be
fabricated which have high strength and excellent compatibility
with most existing semiconductor fabrication processes. Thus
nanoporous dielectrics offer a viable low-dielectric constant
replacement for common semiconductor dielectrics such as dense
silica.
The preferred method for forming nanoporous dielectrics is through
the use of sol-gel techniques. The word sol-gel does not describe a
product but a reaction mechanism whereby a sol, which is a
colloidal suspension of solid particles in a liquid, transforms
into a gel due to growth and interconnection of the solid
particles. One theory is that through continued reactions within
the sol, one or more molecules in the sol may eventually reach
macroscopic dimensions so that it/they form a solid network which
extends substantially throughout the sol. At this point (called the
gel point), the substance is said to be a gel. By this definition,
a gel is a substance that contains a continuous solid skeleton
enclosing a continuous liquid phase. As the skeleton is porous, the
term "gel" as used herein means an open-pored solid structure
enclosing a pore fluid.
One method of forming a sol is through hydrolysis and condensation
reactions, which can cause a multifunctional monomer in a solution
to polymerize into relatively large, highly branched particles.
Many monomers suitable for such polymerization are metal alkoxides.
For example, a tetraethoxysilane (TEOS) monomer may be partially
hydrolyzed in water by the reaction
Reaction conditions may be controlled such that, on the average,
each monomer undergoes a desired number of hydrolysis reactions to
partially or fully hydrolyze the monomer. TEOS which has been fully
hydrolyzed becomes Si(OH).sub.4. Once a molecule has been at least
partially hydrolyzed, two molecules can then link together in a
condensation reaction, such as
or
to form an oligomer and liberate a molecule of water or ethanol.
The Si--O--Si configuration in the oligomer formed by these
reactions has three sites available at each end for further
hydrolysis and condensation. Thus, additional monomers or oligomers
can be added to this molecule in a somewhat random fashion to
create a highly branched polymeric molecule from literally
thousands of monomers. An oligomerized metal alkoxide, as defined
herein, comprises molecules formed from at least two alkoxide
monomers, but does not comprise a gel.
Sol-gel reactions form the basis for xerogel and aergel film
deposition. In a typical thin film xerogel process, an ungelled
precursor sol may be applied (e.g., spray coated, dip-coated, or
spin-coated) to a substrate to form a thin film on the order of
several microns or less in thickness, gelled, and dried to form a
dense film. The precursor sol often comprises a stock solution and
a solvent, and possibly also a gelation catalyst that modifies the
pH of the precursor sol in order to speed gelation. During and
after coating, the volatile components in the sol thin film are
usually allowed to rapidly evaporate. Thus, the deposition,
gelation, and drying phases may take place simultaneously (at least
to some degree) as the film collapses rapidly to a dense film. In
contrast, an aerogel process differs from a xerogel process largely
by avoiding pore collapse during drying of the wet gel. Some
methods for avoiding pore collapse include wet gel treatment with
condensation-inhibiting modifying agents (as described in Gnade
'802) and supercritical pore fluid extraction.
SUMMARY OF THE INVENTION
Between aerogels and xerogels, aerogels are the preferable of the
two dried gel materials for semiconductor thin film nanoporous
dielectric applications. Typical thin film xerogel methods produce
films having limited porosity (up to 60% with large pore sizes, but
generally substantially less than 50% with pore sizes useful in
submicron semiconductor fabrication). While some prior art xerogels
have porosities greater than 50%; these prior art xerogels had
substantially larger pore sizes (typically above 100 nm). These
large pore size gels have significantly less mechanical strength.
Additionally, their large size makes them unsuitable for filling
small (typically less than 1 mm, and potentially less than 100 nm)
patterned gaps on a microcircuit and limits their optical film uses
to only the longer wavelengths. A nanoporous aerogel thin film, on
the other hand, may be formed with almost any desired porosity
coupled with a very fine pore size. Generally, as used herein,
nanoporous materials have average pore sizes less than about 25 nm
across, but preferably less than 20 nm (and more preferably less
than 10 nanometers and still more preferably less than 5
nanometers). In many formulations using this method, the typical
nanoporous materials for semiconductor applications may have
average pore sizes at least 1 nm across, but more often at least 3
nm. The nanoporous inorganic dielectrics include the nanoporous
metal oxides, particularly nanoporous silica.
In many nanoporous thin film applications, such as aerogels and
xerogels used as optical films or in microelectronics, the precise
control of film thickness and aerogel density are desirable.
Several important properties of the film are related to the aerogel
density, including mechanical strength, pore size and dielectric
constant. It has now been found that both aerogel density and film
thickness are related to the viscosity of the sol at the time it is
applied to a substrate. This presents a problem which was
heretofore unrecognized. This problem is that with conventional
precursor sols and deposition methods, it is extremely difficult to
control both aerogel density and film thickness independently and
accurately.
Nanoporous dielectric thin films may be deposited on patterned
wafers, often over a level of patterned conductors. It has now been
recognized that sol deposition should be completed prior to the
onset of gelation to insure that gaps between such conductors
remain adequately filled and that the surface of the gel remains
substantially planar. To this end, it is also desirable that no
significant evaporation of pore fluid occur after gelation, such as
during aging. Unfortunately, it is also desirable that the gel
point be reachable as soon after deposition as possible to simplify
processing, and one method for speeding gelation of thin films is
to allow evaporation to occur. It is recognized herein that a
suitable precursor sol for aerogel deposition should allow control
of film thickness, aerogel density, gap fill and planarity, and be
relatively stable prior to deposition, and yet gel relatively soon
after deposition and age without substantial evaporation.
A method has now been found which allows controlled deposition of
aerogel thin films from a multi-solvent precursor sol. In this
method, sol viscosity and film thickness may be controlled
relatively independently. This allows film thickness to be rapidly
changed from a first known value to a second known value which can
be set by solvent ratios and spin conditions, thus keeping film
thickness largely independent of aerogel density and allowing rapid
gelation. However, at the same time, the solid:liquid ratio present
in the film at drying (and therefore the aerogel density) can be
accurately determined in the precursor sol prior to deposition,
independent of spin conditions and film thickness.
Even with this novel separation of the deposition problem into
viscosity control and density control subproblems, our experience
has been that thin film sol-gel techniques for forming xerogels and
aerogels generally require some method, such as atmospheric
control, to limit evaporation before drying, such as after gelation
and during aging. In principle, this evaporation rate control can
be accomplished by controlling the solvent vapor concentration
above the wafer. However, our experience has shown that the solvent
evaporation rate is very sensitive to small changes in the vapor
concentration and temperature. In an effort to better understand
this process, we have modeled the isothermal vaporization of
several solvents from a wafer as a function of percent saturation.
The ambient temperature evaporation rates for some of these
solvents are given in FIG. 1. For evaporation to not be a
processing problem, the product of the evaporation rate and
processing time (preferably on the order of minutes) should be
significantly less than the film thickness. This suggests that for
solvents such as ethanol, the atmosphere above the wafer would have
to be maintained at over 99% saturation. However, there can be
problems associated with allowing the atmosphere to reach
saturation or supersaturation. Some of these problems are related
to condensation of an atmospheric constituent upon the thin film.
Condensation on either the gelled or ungelled thin film has been
found to cause defects in an insufficiently aged film. Thus, it is
generally desirable to control the atmosphere such that no
constituent is saturated.
Rather than using a high volatility solvent and precisely
controlling the solvent atmosphere, we have discovered that a
better solution is to use a low volatility solvent with less
atmospheric control. Upon investigating-this premise, we have
discovered that glycerol makes an excellent solvent.
The use of glycerol allows a loosening (as compared to prior art
solvents) of the required atmospheric control during deposition,
gelation, and/or aging. This is because, that even though
saturation should still preferably be avoided, the atmospheric
solvent concentration can be lowered without excessive evaporation.
FIG. 2 shows how the evaporation rate of glycerol varies with
temperature and atmospheric solvent concentration. It has been our
experience that, with glycerol, acceptable gels can be formed by
depositing, gelling and aging in an uncontrolled or a substantially
uncontrolled atmosphere.
In the production of nanoporous dielectrics it is preferable to
subject the wet gel thin film to a process known as aging.
Hydrolysis and condensation reactions do not stop at the gel point,
but continue to restructure, or age, the gel until the reactions
are purposely halted. It is believed that during aging,
preferential dissolution and redeposition of portions of the solid
structure produce beneficial results, including higher strength,
greater uniformity of pore size, and a greater ability to resist
pore collapse during drying. Unfortunately, we have now found that
conventional aging techniques used for bulk gels are poorly suited
for aging thin films in semiconductor processing, partly because
they generally require liquid immersion of the substrate and partly
because they require days or even weeks to complete. One aspect of
this invention includes a vapor phase aging technique that avoids
liquid immersion or premature drying of the wet gel thin film and
that, surprisingly, can age such a thin film in a matter of
minutes.
Again, aerogels are nanoporous materials which can be used for a
variety of purposes including as films or in bulk. It should be
noted, however, the problems incurred in film fabrication
processing is so different from bulk processing problems, that, for
practical purposes, film processing is not analogous to bulk
processing.
Generally, we have now found that aging in a saturated atmosphere
avoids the difficulties encountered with liquid immersion aging.
Furthermore, this aspect of the invention provides several
approaches for aging wet gels at increased temperatures. These
methods may be used even when the wet gel originally contains low
boiling point pore liquids. However, they work better with low
volatility solvents. Finally, this aspect of the invention provides
for adding an optional vapor phase aging catalyst to the aging
atmosphere to speed aging.
Aging a wet gel in thin film form is difficult, as the film
contains an extremely small amount of pore fluid that should be
held fairly constant for a period of time in order for aging to
occur. If pore fluid evaporates from the film before aging has
strengthened the network, the film will tend to densify in xerogel
fashion. On the other hand, if excess pore fluid condenses from the
atmosphere onto the thin film before the network has been
strengthened, this may locally disrupt the aging process and cause
film defects.
Thus, we now know that some method of pore fluid evaporation rate
control during aging is beneficial to aerogel thin film
fabrication. In principle, evaporation rate control during aging
can be accomplished by actively controlling the pore fluid vapor
concentration above the wafer. However, the total amount of pore
fluid contained in, for instance, a 1 mm thick 70% porous wet gel
deposited on a 150 mm wafer is only about 0.012 mL, an amount that
would easily fit in a single 3 mm diameter drop of fluid. Typical
thin films used for nanoporous dielectrics on semiconductor wafers
are approximately 1000 times thinner. Thus, actively controlling
the pore fluid vapor concentration (by adding or removing solvent
to the atmosphere) to allow no more than, e.g., 1%, or less, pore
fluid evaporation during aging presents a difficult proposition;
the surface area of the thin film is high and the allowable
tolerance for pore fluid variations is extremely small. In
particular, evaporation and condensation control are especially
important for rapid aging at elevated temperature, where film
production processes have heretofore apparently not been
practically possible.
We have overcome the evaporation rate control problem by not
attempting to actively control pore fluid vapor concentration above
a wafer at all. Instead, the wafer is processed in an extremely
low-volume chamber, such that through natural evaporation-of a
relatively small amount of the pore fluid contained in the wet gel
film, the processing atmosphere becomes substantially saturated in
pore fluid. Unless the wafer is cooled at some point in a
substantially saturated processing atmosphere, this method also
naturally avoids problems with condensation, which should generally
be avoided, particularly during high temperature processing.
A method for forming a thin film nanoporous dielectric on a
semiconductor substrate is disclosed herein. This method comprises
the steps of providing a semiconductor substrate and depositing an
nanoporous aerogel precursor sol upon the substrate. This aerogel
precursor sol comprises a metal-based aerogel precursor reactant
and a first solvent comprising glycerol; wherein, the molar ratio
of the molecules of glycerol to the metal atoms in the reactant is
at least 1:16. The method further comprises allowing the deposited
sol to create a gel, wherein the gel comprises a porous solid and a
pore fluid; and forming a dry, nanoporous dielectric by removing
the pore fluid in a drying atmosphere without substantially
collapsing the porous solid. In this method, the pressure of the
drying atmosphere during the forming step is less than the critical
pressure of the pore fluid, preferably near atmospheric
pressure.
Preferably, the aerogel precursor reactant may be selected from the
group consisting of metal alkoxides, at least partially hydrolyzed
metal alkoxides, particulate metal oxides, and combinations
thereof. Preferably, the aerogel precursor reactant comprises
silicon. In some embodiments, the aerogel precursor reactant is
TEOS. Typically, the molar ratio of the molecules of glycerol to
the metal atoms in the reactant is no greater than 12:1, and
preferably, the molar ratio of the molecules of glycerol to the
metal atoms in the reactant is between 1:2 and 12:1. In some
embodiments, the molar ratio of the molecules of glycerol to the
metal atoms in the reactant is between 2.5:1 and 12:1. In this
method, it is also preferable that the nanoporous dielectric has a
porosity greater than 60% and an average pore diameter less than 25
nm. In some embodiments, the aerogel precursor also comprises a
second solvent. Preferably, the second solvent has a boiling point
lower than glycerol's. In some embodiments, the second solvent may
be ethanol. In some embodiments, the first solvent also comprises a
glycol, preferably selected from the group consisting of ethylene
glycol, 1,4-butylene glycol, 1,5-pentanediol, and combinations
thereof. After aging but before drying, in some embodiments, the
aging fluid is replaced by a drying fluid. This allows, e.g.,
rapid, lower temperature (e.g.; room temperature) drying with a
fluid that evaporates faster and has a suitably low surface
tension. Examples of drying fluids include heptane, ethanol,
acetone, 2-ethylbutyl alcohol and some alcohol-water mixtures.
Thus, this invention allows controlled porosity thin film
nanoporous aerogels to be deposited, gelled, aged, and dried
without atmospheric controls. In another aspect, this invention
allows controlled porosity thin film nanoporous aerogels to be
deposited, gelled, rapidly aged at an elevated temperature, and
dried with only passive atmospheric controls, such as limiting the
volume of the aging chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention, including various features and advantages
thereof, may be best understood with reference to the following
drawings, wherein:
FIG. 1 contains a graph of the variation of evaporation rate with
saturation ratio and solvent type.
FIG. 2 contains a graph of the evaporation rate for glycerol a
function of temperature and atmospheric saturation ratio.
FIG. 3 contains a graph of the theoretical relationship between
porosity, refractive index, and dielectric constant for nanoporous
silica dielectrics.
FIG. 4 contains a graph of the change in gel times (without solvent
evaporation) for bulk ethylene glycol-based gels as a function of
base catalyst
FIG. 5 contains a graph of the variation of modulus with density
for a non-glycol-based gel and an ethylene glycol-based gel.
FIG. 6 contains a graph showing the distribution of pore sizes of a
bulk glycerol-based nanoporous dielectric according to the present
invention.
FIG. 7 contains a graph of the evaporation rate for ethylene glycol
as a function of temperature and atmospheric saturation ratio.
FIG. 8 contains a graph showing the change in vapor pressure with
temperature.
FIG. 9 contains a graph showing the shrinkage of a thin film when
dried in a 5 mm thick container.
FIG. 10 contains a graph showing the shrinkage of a thin film when
dried in a 1 mm thick container.
FIGS. 11A-11B contain graphs of the viscosity variation as a
function of alcohol volume fraction for some ethylene
glycol/alcohol and glycerol/alcohol mixtures.
FIGS. 12A-12B contain cross-sections of a semiconductor substrate
at several points during deposition of a thin film according to the
present invention.
FIG. 13 is a flow chart of a deposition process for a nanoporous
dielectric according to the present invention.
FIG. 14 contains a graph of the theoretical molar ratio of glycerol
molecules to metal atoms vs. porosity of a nanoporous dielectric
according to the present invention.
FIG. 15 contains a graph of relative film thickness and relative
film viscosity as a function of time for one embodiment of the
present invention.
FIGS. 16A and 16B contain, respectively, a cross-sectional and a
plan view of a sol-gel thin film processing apparatus according to
the present invention.
FIG. 16C contains a cross-sectional view of the same apparatus in
contact with a substrate.
FIGS. 17A and 17B contain, respectively, cross-sectional views of
another apparatus according to the present invention, empty and
enclosing a substrate.
FIGS. 18A and 18B contain, respectively, cross-sectional views of
yet another apparatus according to the present invention, empty and
enclosing a substrate.
FIGS. 19A, 19B and 19C contain cross-sectional views of additional
apparatus configurations which illustrate other aspects of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Typical sol-gel thin film processes produce gels which collapse and
densify upon drying, thus forming xerogels having only a few
percent porosity. Under the uncontrolled drying conditions of
xerogel film formation, it has been neither critical nor possible
to completely separate the deposition, aggregation, gelation, and
drying steps during formation of the thin film, as the entire
process may be completed in a few seconds. However, it has now been
found that such methods are generally unsuited for depositing high
porosity thin films with a controllable low density; because in an
aerogel type drying process, the film remains substantially
undensified after drying, its final density is largely determined
by the solid:liquid ratio in the film at the gel time. It has now
been discovered that the following criteria are desirable for
aerogel thin film deposition, particularly where the thin film is
required to planarize and/or gap fill a patterned wafer:
1) an initial viscosity suitable for spin-on application
2) stable viscosity at deposition
3) stable film thickness at gel time
4) a predetermined solid:liquid ratio at gel time
5) gelation shortly after deposition
No prior art precursor sol and method have been found which meet
these conditions. However, in accordance with the present
invention, it has now been found that a sol prepared with at least
two solvents in specific ratios may be used to meet these
conditions.
The method of depositing and gelling such a precursor sol can be
best understood with reference to FIG. 15.
As shown in FIG. 15 for time t=0, a multi-solvent precursor sol may
be spun onto a wafer at an initial film thickness D0 and an initial
viscosity h0. This is preferably done in a controlled atmosphere
having a partial pressure of the low volatility solvent which
greatly retards evaporation of the low volatility solvent from the
wafer. Thus after spin-on application, the high volatility solvent
is preferentially removed from the wafer during evaporation time
period T1 while the low volatility solvent is maintained, thereby
decreasing the film thickness to D1. Viscosity also changes during
this time to h1, preferably due primarily to the removal of
solvent. Ideally, little cross-linking of polymeric clusters in the
sol occurs during this time. At the end of T1, substantially all of
the high volatility solvent should be evaporated, at which time
film thickness should stabilize or proceed to shrink at a much
reduced rate, thereby providing a predetermined liquid:solid ratio
and thickness for the thin film at gel time.
Time period T2 has the primary purpose of providing separation
between the endpoint of evaporation time period T1 and the gel
point which occurs during gelation time period T3. Preferably, time
period T2 is greater than 0. However, some precursors, particularly
those with solvents such as glycerol, that promote faster gelation,
will gel toward the end of period T1. Additionally, during time
period T1 or T2 a vapor-phase catalyst such as ammonia may be
introduced into the controlled atmosphere. This catalyst may
diffuse into the thin film, further activating the sol and
promoting rapid cross-linking. Although little or no evaporation
preferably. takes place during T2, viscosity should begin to
increase substantially as cross-linking continues to link polymeric
clusters.
Evaporation after the gel point may result in poor gap-fill and
planarity for patterned wafers. Consequently, after gelation time
period T3, film thickness is preferably held nearly constant until
the gel point has passed by limiting evaporation. Sometime during
time period T3, a marked change in viscosity occurs as the sol
nears the gel point, where large polymeric clusters finally join to
create a spanning cluster which is continuous across the thin
film.
Several advantages of this new approach are apparent from FIG. 15.
Sol viscosity and film thickness are both allowed to change
rapidly, but generally not at the same time. Also, film thickness
is changed from a first known value to a second known value which
can be independently set by solvent ratios and spin conditions.
Using this method, a low viscosity film may be applied, quickly
reduced to a preset thickness, and rapidly gelled at a desired
density.
The preceding paragraphs teach a method of varying the precursor
sol viscosity independently of the dried gel density. However, it
still leaves open the question of which solvents are most
appropriate. Our experience shows that the solvent evaporation rate
for traditional aerogel solvents is very sensitive to small changes
in the vapor concentration and temperature. In an effort to better
understand this process, we have modeled isothermal solvent
vaporization from a wafer as a function of percent saturation. This
modeling is based on mass transfer theory. Transport Phenomena,
(particularly Chapters 16 and 17) by R. B. Bird, W. E. Stewart, and
E. N. Lighffoot, is a good reference for mass transfer theory.
These calculations were performed for a range of solvents. The
ambient temperature evaporation rates for some of these solvents
are given in FIG. 1. For evaporation to not be a processing
problem, the product of the evaporation rate and processing time
(preferably on the order of minutes) should be significantly less
than the film thickness. This suggests that for solvents such as
ethanol, the atmosphere above the wafer would have to be maintained
at over about 99% saturation. However, there can be problems
associated with allowing the atmosphere to reach saturation or
supersaturation. Some of these problems are related to condensation
of an atmospheric constituent upon the thin film. Condensation on
either the gelled or ungelled thin film has been found to cause
defects in an insufficiently aged film. Thus, it is generally
desirable to control the atmosphere such that no constituent is
saturated.
Rather than using a high volatility solvent and precisely
controlling the solvent atmosphere, we have discovered that a
better solution is to use a low volatility solvent with less
atmospheric control. Upon investigating this premise, we have
discovered that glycerol makes an excellent solvent.
The use of glycerol allows a loosening (as compared to prior art
solvents) of the required atmospheric control during deposition
and/or gelation. This is because, that even though saturation
should still preferably be avoided, the atmospheric solvent
concentration can be lowered without excessive evaporation. FIG. 2
shows how the evaporation rate of glycerol varies with temperature
and atmospheric solvent concentration. It has been our experience
that, with glycerol, acceptable gels can be formed by depositing
and gelling in an uncontrolled or a substantially uncontrolled
atmosphere. In this most preferred approach, (a substantially
uncontrolled atmosphere) atmospheric controls, if any, during
deposition and gelation are typically limited to cleanroom
temperature and humidity controls, although the wafer and/or
precursor sol may have independent temperature controls.
One attractive feature of using glycerol as a solvent is that at
ambient temperature, the evaporation rate is sufficiently low so
that several hours at ambient conditions will not yield dramatic
shrinkage for thin films. It has been our experience that with
glycerol, acceptable gels can be formed by depositing, gelling, and
aging in an uncontrolled or a substantially uncontrolled
atmosphere. With glycerol, the ambient temperature evaporation rate
is sufficiently low so that several hours at ambient conditions
will not yield dramatic shrinkage for thin films. It has also been
our experience that with ethylene glycol, acceptable gels can be
formed by depositing, and gelling an uncontrolled or a
substantially uncontrolled atmosphere. With ethylene glycol, the
ambient temperature evaporation rate is higher than glycerol, but
still sufficiently low so that several minutes at ambient
conditions will not yield dramatic shrinkage for thin films.
However, the ethylene glycol-based sols have significantly lower
viscosities than comparable glycerol-based sols, thus simplifying
deposition. Also, the pore fluids in glycerol-based sols have
significantly higher surface tensions than comparable ethylene
glycol-based sols, thus making low shrinkage drying more
difficult.
In addition to serving as a low vapor pressure and water-miscible
solvent, ethylene glycol and glycerol may also participate in
sol-gel reactions. Although the exact reactions in this process
have not been fully studied, some reactions can be predicted. If
tetraethoxysilane (TEOS) is employed as a precursor, ethylene
glycol can exchange with the ethoxy groups:
Similarly, if tetraethoxysilane (TEOS) is employed as a precursor
with a glycerol solvent, the glycerol can exchange with the ethoxy
groups:
In principle, the presence and concentration of these chemical
groups can change the precursor reactivity (i.e., gel time), modify
the gel microstructure (surface area, pore size distribution,
etc.), change the aging characteristics, or change nearly any other
characteristic of the gel.
The use of a new solvent system can change a wide range of
processing parameters including gel time, viscosity, aging
conditions and drying shrinkage. Many of these properties such as
gel times are difficult to measure on thin films. Although bulk and
thin film properties may be different, it is often useful to
perform a series of experiments on bulk samples (e.g. approximately
5 mm diameter by 30 mm long) to provide a better understanding of
how changing solvent systems affects the nanoporous silica
process.
Glycerol can react with TEOS and produce a dried gel with
surprisingly different properties than that of an ethanol/TEOS gel.
Unanticipated property changes in the glycerol/TEOS based gels
generally include (at least on most formulations):
Lower density is achievable without supercritical drying or
pre-drying surface modification
Greatly simplified aging
Shorter gel times even without a catalyst
Strengths of bulk samples which are approximately an order of
magnitude greater (at a given density) than conventional TEOS
gels
Very high surface area (.about.1,000 m.sup.2 /g)
High optical clarity of bulk samples (This is likely due to a
narrow pore size distribution)
Low density--With this invention, it is possible to form dried gels
at very low densities without pre-drying surface modification or
supercritical drying. These low densities can generally be down
around 0.3 to 0.2 g/cm.sup.3 (non-porous SiO.sub.2 has a density of
2.2 g/cm.sup.3), or with care, below 0.1 g/cm.sup.3. Stated in
terms of porosity (porosity is the percentage of a structure which
is hollow), this denotes porosities of about 86% and 91% (about 95%
porosity with a density of 0.1 g/cm.sup.3). As shown in FIG. 3,
these porosities correspond to dielectric constants of about 1.4
for the 86% porous, and 1.2 for 91% porous. The actual mechanism
that allows these high porosities is not fully known. However, it
may be because the gels have high mechanical strength, because the
gels do not have as many surface OH (hydroxyl) groups, a
combination of these, or some other factors. This method also seems
to obtain excellent uniformity across the wafer.
If desired, this process can be adjusted (by varying the
TEOS/solvent ratios) to give any porosity from above 90% down to
about 50%. Typical prior art dried gels with small pore sizes
required either supercritical drying or a surface modification step
before drying to achieve these low densities. While some prior art
xerogels have porosities greater than 50%; these prior art xerogels
had substantially larger pore sizes (typically above 100 nm). These
large pore size gels have significantly less mechanical strength.
Additionally, their large size makes them unsuitable for filling
small (typically less than 1 .mu.m) patterned gaps on a
microcircuit. If desired, this process can also be adjusted (by
varying the TEOS/solvent ratios) to provide porosities below 50%.
Porosities down to 20% are possible when care is taken to prevent
premature gelation.
Thus, this invention has enabled a new, simple nanoporous low
density dielectric fabrication method. This new glycerol-based
method allows both bulk and thin film aerogels to be made without
supercritical drying, or a surface modification step before drying.
Prior art aerogels have required at least one of these steps to
prevent substantial pore collapse during drying.
Density Prediction--By varying the ratio of glycerol to silicon (or
other metal), the density after drying can be accurately predicted.
This accuracy is likely due to the well controlled evaporation
allowed by the low volatility glycerol solvent. As our process
shows excellent shrinkage control during aging and drying, this
allows accurate prediction of the density (and thus porosity) of
the dried gel. Although density prediction had not generally been
considered a large problem with bulk gels, it had typically been
difficult to predict the final porosity of thin film gels. This
accurate density prediction, even for low porosity dried gels, is
one reason why this new process might be preferred over existing
xerogel processes for forming low porosity gels.
Simplified Aging--We have found that in the production of
nanoporous dielectrics it is preferable to subject the wet gel thin
film to a process known as aging. Hydrolysis and condensation
reactions do not stop at the gel point, but continue to
restructure, or age, the gel until the reactions are purposely
halted. It is believed that during aging, preferential dissolution
and redeposition of portions of the solid structure produce
beneficial results. These beneficial results include higher
strength, greater uniformity of pore size, and a greater ability to
resist pore collapse during drying. However, aging a wet gel in
thin film form is difficult, as the film contains an extremely
small amount of pore fluid that should be held fairly constant for
a period of time in order for aging to occur. If pore fluid
evaporates from the film before aging has strengthened the network,
the film will tend to densify in xerogel fashion. On the other
hand, if excess pore fluid condenses from the atmosphere onto the
thin film before the network has been strengthened, this may
locally disrupt the aging process and cause film defects.
Our new, glycerol-based process has radically simplified aging of
thin film nanoporous dielectrics. Other thin film nanoporous
dielectric aging processes have either allowed significant
evaporation, fluid condensation, or required a controlled aging
atmosphere. During deposition and gelation, at least to some
degree, these glycerol-based processes behave similarly to the
ethylene glycol-based processes described below. However, the
ethylene glycol-based gels typically require atmospheric controls
to prevent significant evaporation during aging, even at room
temperature. In contrast, the glycerol-based gels have dramatically
lower evaporation and shrinkage rates during aging. This allows
atmospheric control to be loosened or eliminated during aging. We
can fabricate high quality, thin film, glycerol-based nanoporous
dielectrics with only passive atmospheric controls during room
temperature or high temperature aging.
Shorter Gel Times--The use of glycerol also substantially shortens
the gel time. Many typical ethanol-based precursors have gel times
of at least 400 seconds, when catalyzed (much longer w/o
catalysis). However, we discovered that some glycerol-based
precursors will gel during wafer spin-on, even without catalysis.
This quick gelation is not only faster than an ethanol-based gel,
but also surprisingly faster than an ethylene glycol-based gel.
FIG. 4 shows gel times for two different ethylene glycol-based
compositions as a function of the amount of ammonia catalyst used.
These gel times are for bulk gels for which there is no evaporation
of ethanol and/or water as there would be for thin films.
Evaporation increases the silica content and thus, decreases the
gel time. Therefore, these gel times may be the upper limit for a
given precursor/catalyst. The gel times reported in FIG. 4 are
approximately an order of magnitude shorter than conventional
ethanol-based precursors. Gel times generally also exhibit a first
order dependence on the concentration of ammonia catalyst. This
implies that it may be possible to easily control the gel
times.
For thin films of these new glycerol-based gels, it is routine to
obtain gelation within seconds, even without a gelation catalyst.
We have identified several mechanisms that can be used to launch
gelation in thin films, without the addition of a catalyst. One
method is the concentration of the precursor sol by allowing a
volatile solvent to evaporate. Another method is increasing the pH
by allowing an acid in the precursor sol to evaporate. This
evaporative basification relies on increasing the precursor sol pH
to help initiate gelation. However, this basification process does
not typically require a pH change from below 7 to above 7. This
evaporative basification acts similarly to a typical base catalysis
process, greatly speeding gelation. At room temperature and
pressure, some acids, such as nitric acid, have evaporation rates
comparable to ethanol. Varying the concentrations and/or types of
the high volatility solvent(s) and/or stabilizing acid provides a
simple, yet tremendous flexible method for adjusting the gelation
time.
Higher Strength--The properties of the glycerol-based samples
appear to be quite different from regular gels as evidenced by both
their low degree of drying shrinkage and differences in qualitative
handling of the wet and dry gels. Thus, upon physical inspection,
the glycerol-based dried gels seem to have improved mechanical
properties as compared to both conventional and ethylene
glycol-based dried gels. FIG. 5 shows the bulk modulus measured
during isostatic compaction measurements of one sample prepared
using one ethylene glycol-based and one conventional ethanol-based
dried bulk gel (both have the same initial density). After initial
changes attributed to buckling of the structure, both samples
exhibit power law dependence of modulus with density. This power
law dependence is usually observed in dried gels. However, what is
surprising is the strength of the ethylene glycol-based dried gel.
At a given density (and thus, dielectric constant), the modulus of
this sample of the ethylene glycol dried gel is an order of
magnitude higher than the conventional dried gel. Preliminary
evaluations show that the glycerol-based gels are even stronger
than the ethylene glycol-based gels. These evaluations include
qualitative handling tests and information based on the shrinkage
during drying. The reasons for this strength increase are not
totally clear. However, preliminary experiments indicate that our
rapid gelation times and/or narrow pore size distribution may be
responsible for the high strengths.
High surface area--We measured the surface areas of some dried bulk
gels. These surface areas were on the order of 1,000 m.sup.2 /g, as
compared to our typical ethanol-based dried gels which have surface
areas in the 600-800 m.sup.2 /g range. These higher surface areas
may imply smaller pore size and improved mechanical properties. It
is unclear at this time why these higher surface areas are obtained
with the glycerol-based dried gels.
Pore size distribution--The optical clarity of these dried bulk
gels was greater than any ethanol-based dried gels at this density
that we have previously made. It is possible that this excellent
optical clarity is due to a very narrow pore size distribution.
However, it is unclear why the glycerol has this effect.
Preliminary experiments show that one possible explanation is that
rapid gelation times may be linked to a narrow pore size
distribution. FIG. 6 shows the pore size distribution (as measured
by BJH nitrogen desorption measurements) of a a density of about
0.57 g/cm.sup.3. The mean pore diameter (desorption method) of this
sample was 3.76 nm. As the typical pores are not truly cylindrical,
diameter, as used herein, actually refers to the diameter of an
equivalent cylinder with the same surface area to volume ratio as
the overall gel's surface area to volume ratio.
As shown above, some properties of the glycerol-based gels apply to
both bulk gels and thin films. However, some advantages are most
evident when applied to thin films, such as nanoporous dielectric
films on semiconductor wafers. One important advantage is that this
new method allows high quality nanoporous films to be processed
with no atmospheric controls during deposition or gelation.
Although it is important to be able to deposit and gel thin
nanoporous films without atmospheric controls, it is also desirable
to age thin nanoporous films without atmospheric controls. It has
been discovered that this can present a bigger challenge than
deposition. The primary reason is that while deposition and room
temperature gelation can take place in minutes, or even seconds;
room temperature aging typically requires hours. Thus, an
evaporation rate that provides acceptable shrinkage for a short
process, may cause unacceptable shrinkage when the process times
are lengthened by an order of magnitude.
As an example, we have found that with some glycerol-based gels, a
satisfactory aging time at room temperature is on the order of a
day. However, Table 1 shows that, by using higher temperatures, we
can age thin films with times on the order of minutes. These aging
times are comparable to the preferred aging time of many typical
ethanol-based and ethylene glycol-based gels. Thus, when these
times and temperatures are combined with the evaporation rates of
FIG. 1, FIG. 7, and FIG. 2, they give the approximate thickness
loss during aging as shown in Table 2. These estimated thickness
losses need to be compared with acceptable thickness losses,
particularly for thin film applications. While no firm guidelines
for acceptable thickness loss exist, one proposed guideline, for
some microcircuit applications such as nanoporous dielectrics, is
that the thickness losses should be less than 2% of the film
thickness. For a hypothetical nominal film thickness of 1 .mu.m
(Actual film thicknesses may typically vary from significantly less
than 0.5 .mu.m to several .mu.m thick), this gives an allowable
thickness loss of 20 nm. As shown in Table 2, the glycerol-based
gels can achieve this preliminary goal without atmospheric control
at room temperature. Thus, this invention allows controlled
porosity thin film nanoporous aerogels to be deposited, gelled,
aged, and dried within atmospheric controls. In another aspect,
this invention allows controlled porosity thin film nanoporous
aerogels to be deposited, gelled, rapidly aged at an elevated
temperature, and dried with only passive atmospheric controls, such
as limiting the volume of the aging chamber.
TABLE 1 ______________________________________ Approximate Aging
Time as a Function of Temperature For Some Thin Film Glycerol-Based
Gels Aging Temperature Aging Time For Glycerol-Based Gels (Degrees
C.) (Order of Magnitude Approximations)
______________________________________ 25 1 day 100 5 minutes 140 1
minute ______________________________________
TABLE 2
__________________________________________________________________________
Approximate Thickness Loss During Aging vs. Saturation Ratio.
Thickness Loss During Aging Ethanol-Based Gel EG-Based Gel
Glycerol-Based Gel Aging Time/ % Saturation % Saturation %
Saturation Temperature 0% 50% 99% 0% 50% 99% 0% 50% 99%
__________________________________________________________________________
1 day/ 8 7 86 17 7 172 13 5 .1 25.degree. C. mm mm .mu.m .mu.m
.mu.m nm nm nm nm 300 sec/ -- -- -- 3 1.2 90 600 420 9 100.degree.
C. .mu.m .mu.m nm nm nm nm 60 sec/ -- -- -- -- -- -- 6 3 60
140.degree. C. .mu.m .mu.m nm
__________________________________________________________________________
Improved yield and reliability considerations may require thickness
losses below 2%, such as less than 0.5% or 0.1%. By using passive
atmospheric control, this invention can be extended to these, and
even lower evaporation losses. This passive control involves
placing the gel in a relatively small closed container, at least
during aging. In this aspect of the invention, evaporation from the
wafer acts to raise the saturation ratio of the atmosphere inside
the closed container. At any given temperature, this evaporation
continues until the partial pressure of the vapor increases enough
to equal the vapor pressure of the liquid. Thus,
solvent/temperature combinations with lower vapor pressure will not
allow as much liquid solvent to evaporate as a higher vapor
pressure combination allows. FIG. 8 shows how vapor pressure varies
with temperature for several solvents. If the container size is
known, the amount of evaporation can be calculated. FIG. 9 shows an
estimate of how thick of layer of solvent could potentially be
evaporated if a 70% porous gel is placed in a 5 mm high cylindrical
container that is the same diameter as the wafer. FIG. 10 shows a
similar estimate for a container with a 1 mm high airspace above
the wafer. These figures show that, with a 5 mm high airspace, the
20 nm preliminary goal is feasible up to 120 degrees C. for
glycerol-based gels, but only up to 50 degrees C. for ethylene
glycol-based gels. With the 1 mm airspace, the 20 nm goal is
feasible all the way up to 150 degrees C. for the glycerol-based
gels, but only up to 80 degrees C. for the ethylene glycol-based
gels. Of course, lower temperature processing allows less
evaporation. Passive evaporation control using the 1 mm containers
allows less than 1 nm of thickness loss (0.1% of a 1 .mu.m thick
film) for the glycerol-based gels, even at 100 degrees C.
There are many variations on this passive control approach. One
variation allows the container size to increase. The thickness loss
will linearly increase with the container volume. However, even a
1000 cubic centimeter container typically allows only 5 nm of
glycerol evaporation at 80 degrees C. Another variation is the gel
porosity. Higher porosity gels generally experience greater
thickness losses while lower porosity gels generally experience
slightly smaller thickness losses.
One disadvantage of glycerol is its relatively high viscosity which
can cause problems with gap-filling and/or planarization. As
described above, a low viscosity, high volatility solvent can be
used to lower the viscosity. FIG. 11A shows the calculated
viscosity of some ethanol/glycerol and methanol/glycerol mixtures
at room temperature. As the figure shows, alcohol can significantly
reduce the viscosity of these mixtures. FIG. 11B shows the
calculated viscosity of some ethanol/ethylene glycol and
methanol/ethylene glycol mixtures at room temperature. As this
figure shows, the ethylene glycol is much less viscous than the
glycerol, and small quantities of alcohol significantly reduce the
viscosity of these mixtures. Also, if the viscosity using ethanol
in the stock solution is higher than desired, further improvement
can be realized by employing methanol in the precursor solution.
The viscosities reported in FIGS. 11A-11B are for pure fluid
mixtures only. In fact, depending upon the film precursor solution,
the precursor solution might contain glycerol, alcohol, water, acid
and partially reacted metal alkoxides. Of course, the viscosity can
be increased before deposition by catalyzing the condensation
reaction and hence, the values reported in FIGS. 11A-11B represent
lower bounds.
This multi-solvent approach may be combined with or replaced by an
alternative approach. This alternate approach use elevated
temperatures to reduce the sol viscosity during application. By
heating and/or diluting the precursor during deposition, (such as
by heating the transfer line and deposition nozzle of a wafer spin
station) the viscosity of the precursor sol can be substantially
lowered. Not only does this preheat lower the sol viscosity, it
will also speed gel times and accelerate the evaporation of any
high volatility solvents. It may also be desirable to preheat the
wafer. This wafer preheat should improve process control and may
improve gap fill, particularly for the more viscous precursors.
However, for many applications, wafer preheat is not required, thus
simplifying process flows. When using a spin-on application method
with this no wafer preheat approach, the spin station would not
require a temperature controlled spinner.
Dried gels produced with this simple thin film aerogel fabrication
process can be used in many applications. Some of these uses may
not have been cost effective using prior art methods. These uses
include low dielectric constant thin films (particularly on
semiconductor substrates), miniaturized chemical sensors, thermal
isolation structures, and thermal isolation layers (including
thermal isolation structures for infrared detectors). As a general
rule, many low dielectric constant thin films prefer porosities
greater than 60%, with critical applications preferring porosities
greater than 80 or 90%, thus giving a substantial reduction in
dielectric constant. However, structural strength and integrity
considerations may limit the practical porosity to no more than
90%. Some applications, including thermal isolation structures and
thermal isolation layers, may need to sacrifice some porosity for
higher strength and stiffness. These higher stiffness requirements
may require dielectrics with porosities as low as 30 or 45%. In
other high strength/toughness applications, especially sensors,
where surface area may be more important than density, it may be
preferable to use a low porosity gel with a porosity between 20%
and 40%.
The thin film discussion above has centered around thin film
aerogels for microelectronic circuits. However, aerogels are also
useful in other applications, such as thin films on passive
substrates. These new high strength, easy to fabricate gels now
make many of these uses practical. For purposes of this
application, a passive substrate is defined as a substrate that
does not comprise or contain a microelectronic circuit, or at least
where there is no interaction between the aerogel and the
electronics. Sol-Gel Science by C. J. Brinker and G. W. Scherer
describes several of these uses in chapter 14. These passive uses
may partially include some types of optical coatings, some types of
protective coatings, and some types of porous coatings.
Antireflective (AR) coatings can require a wide range of
porosities. These will typically range from 20% porous to 70%
porous, although higher porosities (above 90%) may be useful where
there is adequate surface protection, and lower porosities (down to
10%, or below) may be useful in high performance coatings or
coatings on substrates with a high index of refraction. In some
single layer AR coatings, it may be preferable to use gels with
porosities between 30% and 55%. Higher performance, multi-layer AR
coatings will prefer denser layers (e.g., porosity between 10% and
30%) next to the substrate, and less dense layers (e.g., porosity
between 45% and 90%) next to the air interface. For higher
strength/toughness applications, especially where high strength and
surface area are the primary goals, it may be preferable to use a
low porosity gel with a porosity between 20% to 40%. Other thin
film coatings may need the lowest density practical, thus needing
porosities greater than 85%, 90%, or even 95%.
There are also many bulk gel applications that can benefit from
these new high strength, easy to fabricate aerogels. These bulk gel
uses include (but are not limited to) nanoporous (e.g., molecular)
sieves, thermal insulation, catalyst supports, adsorbents, acoustic
insulation, and optiseparation membranes. As a general rule, many
bulk uses prefer porosities greater than 60%, with critical
applications preferring porosities greater than 80% or 90%.
However, structural strength and integrity considerations may limit
the practical porosity to no more than 95%. Some applications,
possibly including sieves, may need to sacrifice some porosity for
higher strength and stiffness. These higher stiffness requirements
may require dielectrics with porosities as low as 30 or 45%. In
other high strength/toughness applications, possibly including
catalyst supports and sensors, where surface area may be more
important than density, it may be preferable to use a low porosity
gel with a porosity between 20% and 40%.
Typical sol-gel thin film processes produce gels which collapse and
densify upon drying, thus forming xerogels having limited porosity
(Up to 60% with large pore sizes, but generally substantially less
than 50% with pore sizes of interest). Under the uncontrolled
drying conditions of xerogel film formation, many of the internal
pores permanently collapse. However, in thin film aerogel
formation, the pores remain substantially uncollapsed, even though
there may be a small amount of shrinkage during aging and/or drying
that affects the final density.
Referring now to FIG. 12A, a semiconductor substrate 10 (typically
in wafer form) is shown. Common substrates include silicon,
germanium, and gallium arsenide, and the substrate may include
active devices, lower level wiring and insulation layers, and many
other common structures not shown but known to those skilled in the
art. Several patterned conductors 12 (e.g., of an Al-0.5% Cu
composition) are shown on substrate 10. Conductors 12 typically run
parallel for at least part of their length, such that they are
separated by gaps 13 of a predetermined width (typically a fraction
of a micron). Both the conductors and gaps may have height-to-width
ratios much greater than shown, with larger ratios typically found
in devices with smaller feature sizes.
In accordance with a first embodiment of the present invention, mix
61.0 mL tetraethoxysilane (TEOS), 61.0 mL glycerol, 4.87 mL water,
and 0.2 mL 1M HNO.sub.3 and reflux for 1.5 hours at
.about.60.degree. C. to form a stock solution. Equivalently, mix
0.27 mol TEOS, 0.84 mol glycerol, 0.27 mol water, and 2.04E-4 mol
HNO.sub.3 and reflux for 1.5 hours at .about.60.degree. C. After
the stock solution is allowed to cool, the solution may be diluted
with ethanol to reduce the viscosity. One suitable stock
solution:solvent volume ratio is 1:8. However, this ratio will
depend upon desired film thickness, spin speed, and substrate. This
is mixed vigorously and typically stored in a refrigerator at
.about.7.degree. C. to maintain stability until use. The solution
is typically warmed to room temperature prior to film deposition.
3-5 mL of this precursor sol may be dispensed at room temperature
onto substrate 10, which is then spun at 1500 to 5000 rpm
(depending on desired film thickness) for about 5-10 seconds to
form sol thin film 14. The deposition can be performed in an
atmosphere that has no special control of solvent saturation (e.g.,
in a cleanroom with non-exotic humidity controls). During and after
this deposition and spinning, the ethanol, water, and the nitric
acid are evaporating from film 14, but due to glycerol's low
volatility, no substantial evaporation of the glycerol is
occurring. This evaporation temporarily cools the thin film,
although the film temperature rises within seconds after the
evaporation rate drops off. This cooling retards, but does not
prevent gelation. This evaporation also shrinks thin film 14 and
concentrates the silica content of the sol, forming reduced
thickness film 18. FIG. 12B shows a reduced thickness sol film 18
obtained after substantially all (about 95% or more) of the ethanol
has been removed. This concentrating, evaporative basification,
and/or rewarming of the film typically cause gelation within
seconds.
Film 18 has an approximately known ratio of silicon to pore fluid
at the gel point. This ratio is approximately equal to the ratio of
TEOS to glycerol in the asdeposited sol (with minor changes due to
remaining water, continued reactions and incidental evaporation).
As this method largely prevents the gel from permanently
collapsing, this ratio determines the density of the aerogel film
that will be produced from the sol thin film.
After gelation, the thin film wet gel 18 comprises a porous solid
and a pore fluid, and can preferably be allowed time to age at one
or more controlled temperatures, e.g., about a day at room
temperature. It should be noted that the pore fluid changes
somewhat during processing. These changes may be due to continued
reactions, evaporation/condensation, or chemical additions to the
thin film. Aging may preferably be accomplished by letting the
substrate and gel sit for approximately 24 hours at about
25.degree. C. or by heating it to 130-150.degree. C. for about 1
minute in a closed container.
Aged film 18 may be dried without substantial densification by one
of several methods, including supercritical fluid extraction.
However, with these new glycerol-based gels, one alternative is to
use a solvent exchange to replace the aging fluid with a drying
fluid and then air dry the film 18 from this drying fluid. This
drying method uses a solvent exchange to replace the aging fluid
with a different fluid. Whether this fluid is identical to the
aging fluid or not, the pore fluid that is present during drying is
sometimes referred to as "drying fluid". If used, the solvent
exchange replaces the aging fluid that is dominated by the glycerol
and its associated high surface tension with a drying fluid that
has a lower surface tension. This solvent exchange may be carried
out as a one or two step process. In the two step process, the
first step replaces the aging fluid with an intermediate by
dispensing approximately 3-8 mL of ethanol at room temperature (or
warmer) onto aged thin film 18, then spinning the wafer between
approximately 50 and 500 rpm for about 5-10 seconds. It sometimes
requires between 3 and 6 spin-on sequences to replace most of the
aging fluid. The second step preferably replaces the intermediate
fluid with a drying fluid such as heptane. This step preferably
comprises dispensing approximately 3-8 mL of heptane at room
temperature (or warmer) onto aged thin film 18, then spinning the
wafer between approximately 50 and 500 rpm for about 5-10 seconds.
It sometimes requires between 3 and 6 spin-on sequences to replace
most of the intermediate fluid. This solvent exchange method allows
us to remove nearly all the glycerol-containing fluid before
drying. The drying fluid (heptane in this case) is finally allowed
to evaporate from the wet gel 18, forming a dry nanoporous
dielectric (dried gel). If the film can be satisfactorily dried
from a liquid that is soluble with the aging fluid, the
intermediate may not be required. In many cases, the wet gel can be
dried directly from ethanol, or other suitable solvent.
This evaporation may be performed by exposing the wafer surface to
an atmosphere that is not near saturated with the drying fluid. For
example, the wafer could be in a substantially uncontrolled
atmosphere, or a drying gas could be introduced into the
atmosphere. To prevent boiling, drying should preferably start at a
temperature somewhat below the boiling point of the drying fluid,
such as room temperature. If a higher boiling point drying fluid,
such as glycerol, is used (e.g., drying without solvent
replacement), the starting drying temperature can be increased to a
temperature near or equal to the aging temperature. As the thin
film becomes predominately dry (typically within seconds), the
temperature should then be increased above the boiling point of
both the aging fluid and the drying fluid. This method prevents
destructive boiling, yet insures that all fluid is removed.
Glycerol, as well as some other fluids, either decompose at
approximately the same temperature as they boil, or decompose in
lieu of boiling. With these fluids, particularly fluids like
glycerol that can decompose into toxic substances, care should be
taken not to overheat the evaporated fluid or the undried wafer.
After drying, it is preferable to bake the nanoporous dielectric
for a short time (such as 300.degree. for 15 to 60 minutes) to help
remove any residual materials, such as organics, that are in or on
the dielectric. The theoretical dielectric constant (before surface
modification) of this embodiment is 1.3.
In order to reduce the dielectric constant, it is preferable to
dehydroxylate (anneal) the dried gel. This may be done by placing
the wafer in a dry atmosphere with an agent, such as
hexamethyldisilazane (HMDS) or hexaphenyldisilazane vapor. The HMDS
will replace much of the water and/or hydroxyls bound to the dried
gel's pore surfaces with methyl groups. This replacement may be
performed at room temperature, or warmer. This replacement can not
only remove water and/or hydroxyls, it can also render the dried
gel hydrophobic (water repelling). The hexaphenyldisilazane will
also remove water and/or hydroxyls and render the dried gel
hydrophobic. However, the phenyl groups have a higher temperature
stability than the methyl groups, at the expense of a slightly
higher dielectric constant.
FIG. 13 contains a flow chart of a general method for obtaining an
aerogel thin film from a precursor sol according to one embodiment
of the present invention. Table 3 is a quick summary of some of the
substances used in this method.
TABLE 3 ______________________________________ Substance Summary
Ref Specific Functional # Example Description Preferred Alternates
______________________________________ 10 Silicon Substrate
Semiconductor substrate, Ge, 26 GaAs, active devices, lower level
layers, glass, plastic, optical substrate 12 Al-0.5% Cu Patterned
Al, Cu, other metals, polysilicon Conductors TEOS Precursor Sol
Other silicon-based metal Reactant alkoxides (TMOS, MTEOS, BTMSE,
etc.), alkoxides of other metals, particulate metal oxides, organic
precursors, and combinations thereof Glyercol Precursor Sol
Ethylene glycol, Other trihydric First Solvent alcohols,
combinations of glycerol (Low volatility) and dihydric and/or
trihydric alcohols. 1,4-butylene glycol and 1,5-pentanediol.
1,2,4-butane- triol; 1,2,3-butanetriol; 2-methyl- propanetriol; and
2-(hydroxy- methyl)-1,3-propanediol; 1-4, 1-4, butanediol; and
2-methyl- 1,3-propanediol. Nitric Acid Precursor Sol Other acids
(HNO.sub.3) Stabilizer Ethanol Precursor Sol Methanol, other
alcohols Second Solvent (High volatility) Ethanol Viscosity
Methanol, other alcohols Reduction Solvent Ammonium Gelation
Ammonia, volatile amine species, Hydroxide Catalyst volatile
fluorine species, and other (NH.sub.4 OH) compounds that will raise
the pH of the deposited sol. As-Gelled Aging Fluid Glycerol,
ethylene glycol, water, Pore Fluid ethanol, other alcohols,
combinations thereof. Aging fluid Drying Fluid Heated aging fluid,
heptane, acetone, isoproponal, ethanol, methanol, 2-ethylbutyl
alcohol, alcohol/water mixtures, ethylene glycol, other liquids
that are miscible with the aging fluid, yet have lower surface
tension than the aging fluid, combinations thereof. Hexa- Surface
hexaphenyldisilazane, trimethyl- methyldi- Modification
methoxysilane, dimethyldi- silazane methoxysilane, trimethylchloro-
(HMDS) silane (TMCS), phenyl compounds and fluorocarbon compounds.
______________________________________
In accordance with a second, higher density, embodiment of the
present invention, mix 150.0 mL TEOS, 61.0 mL glycerol, 150.0 mL
ethanol, 12.1 mL water, and 0.48 mL 1M HNO.sub.3 and reflux for 1.5
hours at .about.60.degree. C. to form a stock solution.
Equivalently, mix 0.67 mol TEOS, 0.84 mol glycerol, 2.57 mol
ethanol, 0.67 mol water, and 4.90E-4 mol HNO.sub.3 and reflux for
1.5 hours at .about.60.degree. C. After the stock solution is
allowed to cool, the solution may be diluted with ethanol to reduce
the viscosity. One suitable stock solution:solvent volume ratio is
1:8. This is mixed vigorously and typically stored in a
refrigerator at .about.7.degree. C. to maintain stability until
use. The solution is warmed to room temperature prior to film
deposition. 3-5 mL of this precursor sol may be dispensed at room
temperature onto substrate 10, which is then spun at 1500 to 5000
rpm (depending on desired film thickness) for about 5-10 seconds to
form sol thin film 14. The deposition can be performed in an
atmosphere that is not solvent controlled (e.g., standard exhausts
in a cleanroom with non-exotic humidity controls). During and after
this deposition and spinning, ethanol (a viscosity reduction
additive and a reaction product from the TEOS and water) and water
is evaporating from film 14, but due to glycerol's low volatility,
no substantial evaporation of the glycerol is occurring. This
evaporation shrinks thin film 14 and concentrates the silica
content of the sol forming reduced thickness film 18. FIG. 12B
shows a reduced thickness sol film 18 obtained after substantially
all (about 95% or more) of the water has been removed. This
concentrating typically causes gelation within minutes.
Further processing generally follows the process described in the
first embodiment. After gelation, the thin film wet gel 18
comprises a porous solid and a pore fluid, and can preferably be
allowed time to age at one or more controlled temperatures. Aged
film 18 may be dried without substantial densification by one of
several methods, including supercritical fluid extraction. However,
with the lower density formulations of these new glycerol-based
gels, it is preferable to perform a non-supercritical drying, such
as a solvent exchange followed by air drying the film 18 from the
drying fluid, as described in the first embodiment. The nanoporous
dielectric can then be subjected to a post-dry bake and/or a
surface modification, as described in the first embodiment. The
theoretical dielectric constant (before surface modification) of
this embodiment is 1.6.
In accordance with a third, higher density, embodiment of the
present invention, mix 208.0 mL TEOS, 61.0 mL glycerol, 208.0 mL
ethanol, 16.8 mL water, and 0.67 mL 1M HNO.sub.3 and reflux for 1.5
hours at .about.60.degree. C. to form a stock solution.
Equivalently, mix 0.93 mol TEOS, 0.84 mol glycerol, 3.56 mol
ethanol, 0.93 mol water, and 6.80E-4 mol HNO.sub.3 and reflux for
1.5 hours at .about.60.degree. C. After the stock solution is
allowed to cool, the solution may be diluted with ethanol to reduce
the viscosity. One suitable stock solution:solvent volume ratio is
1:8. This is mixed vigorously and typically stored in a
refrigerator at .about.7.degree. C. to maintain stability until
use. The solution is warmed to room temperature prior to film
deposition. 3-5 mL of this precursor sol may be dispensed at room
temperature onto substrate 10, which is then spun at 1500 to 5000
rpm (depending on desired film thickness) for about 5-10 seconds to
form sol thin film 14. The deposition can be performed in an
atmosphere that is not solvent controlled (e.g., standard exhausts
in a cleanroom with non-exotic humidity controls). During and after
this deposition and spinning, ethanol and water is evaporating from
film 14, but due to glycerol's low volatility, no substantial
evaporation of the glycerol is occurring. This evaporation shrinks
thin film 14 and concentrates the silica content of the sol forming
reduced thickness film 18. FIG. 12B shows a reduced thickness sol
film 18 obtained after substantially all (about 95% or more) of the
water has been removed. This concentrating typically causes
gelation within minutes.
Further processing generally follows the process described in the
first embodiment. After gelation, the thin film wet gel 18
comprises a porous solid and a pore fluid, and can preferably be
allowed time to age at one or more controlled temperatures. Aging
may be accomplished by letting the device sit for approximately 24
hours at 25.degree. C. Aged film 18 may be dried without
substantial densification by one of several methods, including
supercritical fluid extraction, or a solvent exchange followed by
air drying. However, especially in this higher density formulation
of these new glycerol-based gels, it is preferable to air dry the
film 18 from the aging fluid. In this direct drying method, the
wafer surface is exposed to an atmosphere that is not near
saturated with the drying fluid. A simple method is to remove the
cover from a low volume aging chamber, thus exposing the gel
surface to a substantially uncontrolled atmosphere. Another method
introduces a drying gas into the aging chamber or atmosphere. With
this direct drying method, the starting drying temperature can
preferably be increased to a temperature near or equal to the aging
temperature. This high temperature drying reduces surface tension
and associated shrinkage, speeds drying, and simplifies processing.
As the thin film becomes predominately dry (typically within
seconds for high temperature drying), the temperature should then
be increased above the boiling point of both the aging fluid and
the drying fluid (they are often the same fluid). This method
prevents destructive boiling, yet insures that all fluid is
removed. Since this method's drying fluid comprises glycerol, which
can decompose into toxic substances, care should be taken not to
overheat the evaporated fluid or the undried wafer. The nanoporous
dielectric can then be subjected to a post-dry bake and/or a
surface modification, as described in the first embodiment. The
theoretical dielectric constant (before surface modification) of
this embodiment is 1.76.
In accordance with a fourth embodiment of the present invention,
mix 278.0 mL TEOS, 61.0 mL glycerol, 278.0 mL ethanol, 22.5 mL
water, and 0.90 mL 1M HNO.sub.3 and reflux for 1.5 hours at
.about.60.degree. C. to form a stock solution. Equivalently, mix
1.25 mol TEOS, 0.84 mol glycerol, 4.76 mol ethanol, 1.25 mol water,
and 9.1E-4 mol HNO.sub.3 and reflux for 1.5 hours at
.about.60.degree. C. After the stock solution is allowed to cool,
the solution may be diluted with ethanol to reduce the viscosity.
One suitable stock solution:solvent volume ratio is 1:8. This is
mixed vigorously and typically stored in a refrigerator at
.about.7.degree. C. to maintain stability until use. The solution
is warmed to room temperature prior to film deposition. 3-5 mL of
this precursor sol may be dispensed at room temperature onto
substrate 10, which is then spun at 1500 to 5000 rpm (depending on
desired film thickness) for about 5-10 seconds to form sol thin
film 14. The deposition can be performed in an atmosphere that is
not solvent controlled (e.g., standard exhausts in a cleanroom with
non-exotic humidity controls). During and after this deposition and
spinning, ethanol and water is evaporating from film 14, but due to
glycerol's low volatility, no substantial evaporation of the
glycerol is occurring. This evaporation shrinks thin film 14 and
concentrates the silica content of the sol forming reduced
thickness film 18. FIG. 12B shows a reduced thickness sol film 18
obtained after substantially all (about 95% or more) of the water
has been removed. This concentrating typically causes gelation
within minutes.
Further processing generally follows the process described in the
third embodiment. After gelation, the thin film wet gel 18
comprises a porous solid and a pore fluid, and can preferably be
allowed time to age at one or more controlled temperatures. Aged
film 18 may be dried without substantial densification by one of
several methods, including supercritical fluid extraction. However,
it is preferable to air dry the film 18 from the aging fluid, as
described in the third embodiment. The nanoporous dielectric can
then be subjected to a post-dry bake and/or a surface modification,
as described in the first embodiment. The theoretical dielectric
constant (before surface modification) of this embodiment is
1.96.
In accordance with a fifth embodiment of the present invention, mix
609.0 mL TEOS, 61.0 mL glycerol, 609.0 mL ethanol, 49.2 mL water,
and 1.97 mL 1M HNO.sub.3 and reflux for 1.5 hours at
.about.60.degree. C. to form a stock solution. Equivalently, mix
2.73 mol TEOS, 0.84 mol glycerol, 10.4 mol ethanol, 2.73 mol water,
and 2.00E-3 mol HNO.sub.3 and reflux for 1.5 hours at
.about.60.degree. C. After the stock solution is allowed to cool,
the solution may be diluted with ethanol to reduce the viscosity.
One suitable stock solution:solvent volume ratio is 1:8. This is
mixed vigorously and typically stored in a refrigerator at
.about.7.degree. C. to maintain stability until use. The solution
is warmed to room temperature prior to film deposition. 3-5 mL of
this precursor sol may be dispensed at room temperature onto
substrate 10, which is then spun at 1500 to 5000 rpm (depending on
desired film thickness) for about 5-10 seconds to form sol thin
film 14. The deposition can be performed in an atmosphere that is
not solvent controlled (e.g., standard exhausts in a cleanroom with
non-exotic humidity controls). During and after this deposition and
spinning, ethanol and water is evaporating from film 14, but due to
glycerol's low volatility, no substantial evaporation of the
glycerol is occurring. This evaporation shrinks thin film 14 and
concentrates the silica content of the sol forming reduced
thickness film 18. FIG. 12B shows a reduced thickness sol film 18
obtained after substantially all (about 95% or more) of the water
has been removed. This concentrating typically causes gelation
within minutes.
Further processing generally follows the process described in the
third embodiment. After gelation, the thin film wet gel 18
comprises a porous solid and a pore fluid, and can preferably be
allowed time to age at one or more controlled temperatures. Aged
film 18 may be dried without substantial densification by one of
several methods, including supercritical fluid extraction. However,
it is preferable to air dry the film 18 from the aging fluid, as
described in the third embodiment. The nanoporous dielectric can
then be subjected to a post-dry bake and/or a surface modification,
as described in the first embodiment. The theoretical dielectric
constant (before surface modification) of this embodiment is
2.5.
Other ratios of solvent to reactant ratios can be used to provide
different porosities/dielectric constants. FIG. 14 shows the
theoretical relationship between the molar ratio of glycerol
molecules to metal atoms and the porosity of a nanoporous
dielectric for the case where all ethanol is evaporated from the
deposited sol. Typically, the higher porosity glycerol-based gels
(generally less than about 0.51 g/cc) prefer a solvent exchange or
other method to lessen shrinkage during drying. On the other hand,
the lower porosity gels require care to prevent early gelation.
This may comprise pH adjustment, temperature control, or other
methods known in the art. In some applications, it is also
permissible to allow high volatility solvent evaporation after
gelation.
As described above, the higher density glycerol-based gels
(generally greater than about 0.64 g/cc) can be aged and dried with
little shrinkage, even without a solvent exchange. An unaged wafer
may be placed in a small volume furnace, or a small container,
which can go on a hot plate. After an optional evacuation, the
container is sealed at room temperature. The container remains
sealed as the temperature is ramped up, quickly aging the film, and
lowering the aging/drying fluid viscosity. After sufficient aging
(possibly during the temperature ramp), the gel is ready for
drying. At temperatures near glycerol's boiling point, the glycerol
viscosity can be low enough (compared to the strength of an aged
film of the predetermined porosity), that the glycerol in the
furnace atmosphere can be removed and the film directly dried. Note
that, in the most demanding low density applications, a somewhat
lower surface tension can be obtained by increasing the drying
temperature above the boiling point of glycerol. In these cases,
the furnace needs to withstand the pressure (most sub-critical
drying situations can be handled with pressures under 1 to 3 MPa).
Additionally, care needs to be taken that the glycerol in the
furnace atmosphere is, especially at first, slowly removed. The
glycerol in the furnace atmosphere may be removed, e.g., by
bleeding off the pressure, by a vacuum pump, or by sweeping the
glycerol off with a gas. The furnace temperature may be held
constant or continue to be raised while the glycerol is being
removed (the furnace may be ramped on up to the bake temperature
while sweeping the glycerol off with the gas). While some glycerol
can be introduced during heating to minimize evaporation from the
film, preferably the furnace volume is low enough that evaporation
does not significantly reduce film thickness even without the
introduction of glycerol during heating. If a film requires
supercritical drying, perhaps to eliminate even temporary
shrinkage, it is preferable to use a CO.sub.2 solvent exchange as
is well known in the art.
Although the same stock solutions can be used for bulk aerogels as
thin film aerogels, the processing is substantially different. With
different stock solution mixtures, the following example can be
adapted to provide bulk gels with different porosities. In
accordance with a bulk aerogel embodiment of the present invention,
mix 208.0 mL TEOS, 61.0 mL glycerol, 208.0 mL ethanol, 16.8 mL
water, and 0.67 mL 1M HNO.sub.3 and reflux for 1.5 hours at
.about.60.degree. C. to form a stock solution. Equivalently, mix
0.93 mol TEOS, 0.84 mol glycerol, 3.56 mol ethanol, 0.93 mol water,
and 6.80E-4 mol HNO.sub.3 and reflux for 1.5 hours at
.about.60.degree. C. This is typically stored in a refrigerator at
.about.7.degree. C. to maintain stability until use. The stock
solution is preferably warmed to room temperature prior to placing
into molds. After pouring into molds, the ethanol, water, and acid
is allowed to evaporate, but due to glycerol's low volatility, no
substantial evaporation of the glycerol is occurring. This
evaporation reduces the volume of the stock solution precursor sol
and concentrates the silica content of the sol. It is allowable for
at least some of the evaporation to occur before filling the mold.
This pre-fill evaporation might be especially useful if the
configuration of the mold does not lend itself to substantial
evaporation after filling, such as a low exposed surface area mold
or a mold configuration that is incompatible with shrinkage.
Although this evaporation is not required, it has several
advantages, including faster gelation without a catalyst and less
shrinkage after gelation.
After this evaporation, the sol has an approximately known ratio of
silicon to pore fluid at the gel point. This ratio is approximately
equal to the ratio of TEOS to glycerol in the precursor mix (with
minor changes due to remaining water, continued reactions and
incidental evaporation). As this method largely prevents the gel
from permanently collapsing, this ratio determines the density of
the aerogel that will be produced. If the sol does not gel during
evaporation, this sol will gel soon after substantially all of the
water, ethanol, and acid has evaporated.
Alternatively, one may catalyze the precursor with 0.5M ammonium
nitrate before filling the mold. With this mixture, the sol
typically gels in minutes. Remove the wet gel from the mold and
allow the ethanol and water to evaporate. Typically, the gel will
shrink during this evaporation. However, as with the other
approaches, when the evaporation is substantially complete, the sol
has an approximately known ratio of silicon to pore fluid at the
gel point. This ratio is approximately equal to the ratio of TEOS
to glycerol in the precursor mix (with minor-changes due to
remaining water, continued reactions and incidental evaporation).
As this method largely prevents the gel from permanently
collapsing, this ratio determines the density of the aerogel that
will be produced.
After gelation, the wet gel comprises a porous solid and a pore
fluid, and can preferably be allowed time to age at one or more
controlled temperatures. Aging may preferably be accomplished by
letting the substrate and gel sit for approximately 24 hours at
about 25.degree. C. or by heating it to 130-150.degree. C. for
about 5 minutes in a closed container. These high temperature aging
parameters are valid for a 5 mm diameter bulk aerogel. However, due
to the wet gel's low thermal conductivity, the high temperature
accelerated aging time and temperature combinations are highly
dependent upon the configuration of the bulk gel.
After this initial aging, remove the gel from the molds and dry
directly from the mother liquor (that is, the pore fluid remaining
at the end of aging, with no solvent exchanges for aging or
drying). A slow ramp to and hold at about 500.degree. C. will dry
the gel.
Instead of drying directly from the mother liquor, it may be
preferable, particularly with higher porosity gels, to perform a
solvent exchange. This solvent exchange may be carried out as a one
or two step process. The first step replaces the aging fluid with
an intermediate and the second step preferably replaces the method,
It is preferable to remove the gels from the molds and place it in
sealed tubes containing ethanol and allow a pore fluid exchange for
8 hours at 50 degrees C. At the end of a 8 hour interval, rinse the
gels with ethanol and then store in fresh ethanol in an oven at 50
degrees C. After three to six such intervals, replace the ethanol
with hexane in a similar manner. This solvent exchange method
allows us to remove nearly all the glycerol-containing fluid before
drying. The drying fluid (heptane in this case) is finally allowed
to evaporate from the wet gel, forming a dry aerogel. If the film
can be satisfactorily dried from a liquid that is soluble with the
aging fluid, the intermediate may not be required. In many cases,
the wet gel can be dried directly from ethanol, or other suitable
solvent.
After drying, it is often preferable to bake the aerogel for a
short time (such as 300.degree. for 15 to 60 minutes) to help
remove any residual materials, such as organics, that are in or on
the aerogel. In some applications, it is also desirable to
dehydroxylate (anneal) the dried gel. This may be done by placing
the dry aerogel in a dry atmosphere comprising a surface
modification agent, such as trimethylchlorosilane (TMCS),
hexamethyldisilazane (HMDS), or hexaphenyldisilazane vapor. The
HMDS will replace much of the water and/or hydroxyls bound to the
dried gel's pore surfaces with methyl groups. This replacement can
be performed at room temperature, or warmer. This replacement can
not only remove water and/or hydroxyls, it can also render the
dried gel hydrophobic (water repelling). The hexaphenyldisilazane
will also remove water and/or hydroxyls an hydrophobic. However,
the phenyl groups have a higher temperature stability than the
methyl groups.
In accordance with an ethylene glycol-based embodiment of the
present invention, mix tetraethoxysilane (TEOS), ethylene glycol,
ethanol, water, and acid (1M HNO.sub.3) in a molar ratio of
1:2.4:1.5:1:0.042 and reflux for 1.5 hours at .about.60.degree. C.
After the mixture is allowed to cool, the solution is diluted down
with ethanol to a composition of 70% (by volume) original stock
solution and 30% (by volume) ethanol. This is mixed vigorously and
typically stored in a refrigerator at .about.7.degree. C. to
maintain stability until use. The solution is warmed to room
temperature prior to film deposition. A mixture of stock solution
and 0.25M NH.sub.4 OH catalyst (10:1 volume ratio) is combined and
mixed. 3-5 mL of this precursor sol may be dispensed at room
temperature onto substrate 10, which is then spun at 1500 to 5000
rpm (depending on desired film thickness) for about 5-10 seconds to
form sol thin film 14.
The deposition can be performed in an atmosphere that is
uncontrolled. However, it is preferable to deposit and gel the sol
in a clean room with standard humidity controls. During and after
this deposition and spinning, the ethanol/water mixture is
evaporating from film 14, but due to ethylene glycol's low
volatility, no substantial evaporation of the ethylene glycol is
occurring. This evaporation shrinks thin film 14 and concentrates
the silica content of the sol forming reduced thickness film 18.
FIG. 12B shows a reduced thickness sol film 18 obtained after
substantially all (about 95% or more) of the ethanol has been
removed. This concentrating, combined with the catalyst, typically
causes gelation within minutes or seconds.
Film 18 has an approximately known ratio of silicon to pore fluid
at the gel point. This ratio is approximately equal to the ratio of
TEOS to ethylene glycol in the as-deposited sol (with minor changes
due to remaining water, continued reactions and incidental
evaporation). To the extent that the gel is prevented from
collapsing, this ratio will determine the density of the aerogel
film that will be produced from the sol thin film.
After gelation, the thin film wet gel 18 comprises a porous solid
and a pore fluid, and can preferably be allowed time to age at one
or more controlled temperatures, e.g., about a day at room
temperature. It should be noted that the pore fluid changes
somewhat during processing. These changes may be due to continued
reactions and/or evaporation/condensation. Aging may preferably be
accomplished by letting the device sit in a low volume aging
chamber for approximately 5 minutes at about 100 degrees C.
Aged film 18 may be dried without substantial densification by one
of several methods, including supercritical fluid extraction, or a
solvent exchange followed by air drying. However, it is preferable
to air dry the film 18 from the aging fluid, as described in the
third glycerol embodiment. The nanoporous dielectric can then be
subjected to a post-dry bake and/or a surface modification, as
described in the first glycerol embodiment.
In accordance with another ethylene glycol-based embodiment of the
present invention, mix tetraethoxysilane (TEOS), ethylene glycol,
water, and acid (1M HNO.sub.3) in a molar ratio of 1:4:1:0.042 and
reflux for 1.5 hours at 60.degree. C. This is typically stored in a
refrigerator at .about.7.degree. C. to maintain stability until
use. The solution is preferably warmed to room temperature prior to
film-deposition. 3-5 mL of this precursor sol may be dispensed
(without catalyst) at room temperature onto substrate 10, which is
then spun at 1500 to 5000 rpm (depending on desired film thickness)
for about 5-10 seconds to form sol thin film 14. The deposition can
be performed in an atmosphere that is uncontrolled. However, it is
preferable to deposit and gel the sol in a clean room with standard
humidity controls. During and after this deposition and spinning,
ethanol and water is evaporating from film 14, but due to ethylene
glycol's low volatility, no substantial evaporation of the ethylene
glycol is occurring. This evaporation shrinks thin film 14 and
concentrates the silica content of the sol forming reduced
thickness film 18. FIG. 12B shows a reduced thickness sol film 18
obtained after substantially all (about 95% or more) of the water
has been removed. This concentrating typically causes gelation
within minutes.
After gelation, the thin film wet gel 18 comprises a porous solid
and a pore fluid, and can preferably be allowed time to age at one
or more controlled temperatures, e.g., about a day at room
temperature. It should be noted that the pore fluid changes
somewhat during processing. Aging may preferably be accomplished by
letting the device sit in a low volume aging chamber for
approximately 5 minutes at about 100 degrees C.
Aged film 18may be dried without substantial densification by one
of several methods, including supercritical fluid extraction, or a
solvent exchange followed by air drying. However, it is preferable
to air dry the film 18 from the aging fluid, as described in the
third glycerol embodiment. The nanoporous dielectric can then be
subjected to a post-dry bake and/or a surface modification, as
described in the first glycerol embodiment.
The discussion to this point has shown some of the advantages of
aging in a closed container. Since suitable aging chambers do not
seem to exist, we will describe the chambers we have invented to
implement this process. One embodiment of aging container is
illustrated in FIGS. 16A, 16B and 16C. In this embodiment, a
processing apparatus comprises a body 20, having a substantially
planar plate 22 with a resilient seal 24 attached thereto. Plate 22
need only be planar to the extent necessary to provide clearance
with a thin film during operation, and may be constructed of any
material compatible with the underlying process (e.g.,
semiconductor fabrication), although materials with high thermal
conductivity, such as stainless steel, glass, or aluminum are
preferred. Resilient seal 24 should preferably be designed to
withstand wet gel processing temperatures and pore fluids; many
suitable materials, including TEFLON- and neoprene-based materials,
are known to those of ordinary skill in the art. Depending on the
nature of temperature control used in the apparatus, it may be
preferable to have seal 24 be either substantially thermally
insulating or thermally conductive.
In operation, body 20 may simply be rested on a substrate 26, as
shown in FIG. 16C. This substrate may be an optical substrate, such
as glass or plastic, or a semiconductor substrate, such as a Si
wafer. In this embodiment, seal 24 functions both as an atmospheric
seal and as a spacer which sets the volume of chamber 32 formed by
substrate surface 28, chamber surface 30 and seal 24. For example,
seal 24 may be designed to compress to a thickness of about 1 mm
under the weight of plate 22, thus creating chamber 32 with a 1 mm
height when body 20 is placed on substrate 26. For many thin film
applications, chamber 32 need only be substantially sealed, as some
small degree of vapor leakage over the course of processing
substrate 26 will not appreciably affect the final film
properties.
Body 20 finds application at many points in an aerogel thin film
process. It may be used to limit evaporation before a sol film has
gelled, as an aging chamber for wet gel thin films, as a storage or
transport chamber for such films, or as a drying chamber. In all of
these applications, it is recognized that both sol and gel thin
films contain extremely small amounts of liquid, such that a
chamber of limited volume is necessary to prevent substantial
evaporation from the film.
In another embodiment, body 20 may comprise more elements, as shown
in FIGS. 17A and 17B. In this embodiment, body 20 additionally
comprises a substrate holder 36 and substrate temperature control
means 34. This embodiment shows the additional aspect of a seal 24
located outboard of the substrate (or in some cases seal 24 may
even be deleted), such that a thin film may be formed on the
entirety of substrate surface 28. When chamber 32 is closed, planar
plate 22 and wafer holder 36 may be thermally coupled such that
temperature control means 34 may be used to simultaneously regulate
the temperature of body 20, substrate 26 and chamber 32.
In another embodiment shown in FIGS. 18A and 18B, seal 24 provides
some degree of thermal isolation between planar plate 22 and wafer
holder 36. This allows temperature control means 34 to control
substrate temperature, while separate temperature control means 38
are used to control planar plate temperature. Such an embodiment
may have an advantage for drying a wet gel film, as the temperature
of planar plate 22 can be selectively lowered to promote
condensation on chamber surface 30.
FIGS. 19A, 19B and 19C show additional aspects of these aging
chambers. For example, in FIG. 19A, substrate 26 is shown being
processed in an inverted position. In this embodiment, accidental
or purposeful condensation onto chamber surface 30 may be collected
without the possibility of such condensation dropping onto
substrate surface 26. In FIG. 19B, not only is substrate 26
processed inverted, but a first solvent layer 42 (preferably of the
same composition as at least one pore fluid) is dispensed, e.g.,
from a first solvent supply tube 40, onto chamber surface 30 prior
to closing the chamber. In this embodiment, layer 42 may be used to
help saturate the processing atmosphere, resulting in less
evaporation of pore fluid from substrate 26.
In FIG. 19C, an embodiment is shown wherein some atmospheric
adjustment means 44 is connected through at least one port 46
(which may be closeable) to chamber 32. Atmospheric adjustment
means 44 may be used to create a vacuum or to overpressure chamber
32 as appropriate, or to exchange the atmosphere in chamber 32, or
to supply a pore fluid vapor to chamber 32. This embodiment may be
used, for example, to age a thin film at a temperature higher than
the boiling point of a pore fluid, by operating chamber 32 at above
atmospheric pressure. This embodiment may also be used to remove at
least a portion of the pore fluid vapor from chamber 32 after
aging, thereby allowing the thin film to dry.
Although this invention has been described in terms of several
embodiments, many of these steps may be modified within the scope
of the invention, and other steps can be included to enhance the
overall process. For example, the initial thin film may be
deposited by other common methods, such as dip-coating, flow
coating, or spray-coating instead of spin-coating. Likewise, the
solvent exchange may use dip coating, spray coating, or immersion
in a liquid or vaporous solvent instead of spin-coating. When using
a vaporous solvent, the wafer may be cooled to a temperature lower
than the atmosphere, thus promoting condensation on the wafer.
While water might otherwise be considered a solvent in such a
process, for discussion purposes in this application, water is not
considered a solvent.
Although both glycerol and ethylene glycol each have unique
advantages, there are other low volatility solvents that can be
useful in low shrinkage nanoporous dielectric fabrication. Although
it is preferable to analyze a solvent to determine its expected
evaporation rate, a preliminary preference on the selection of the
low volatility solvent can be made. Nearly all solvents that have a
low evaporation rate at room temperature will have a boiling point
greater than 140.degree. C. Although some solvents with boiling
points less than 140.degree. C. may be useful, a preferred
evaporation rate will typically be found with solvents that have a
boiling point greater than 160.degree. C., and more preferably
greater than 190.degree. C. Solvents with boiling points greater
than 230.degree. C. may also have low enough evaporation rates to
be suitable for deposition and/or aging with little atmospheric
control for short periods of time at 40-80.degree. C. For
processing at 100-150.degree. C. with little atmospheric control,
it is preferable to use solvents with boiling points greater than
270.degree. C. This gives some rough preferences on the lower limit
of preferred boiling points. There are also rough preferences on
the upper limit of preferred boiling points. Most solvents with
boiling points greater than 500.degree. C. will be so viscous that
they require extra care during processing. Typically, the more
useful solvents will have boiling points less than 350.degree. C.,
and preferably less than 300.degree. C. If it is not convenient to
dilute or heat the sol during deposition, it may even be preferable
to use a low volatility solvent with a boiling point less than
250.degree. C. If no one solvent gives all the desired properties,
two or more may be mixed to improve the performance. Thus, our
initial preliminary preference on the selection of the low
volatility solvent is a boiling point in the 175-250.degree. C.
range and (for TEOS based gels) that it be miscible with both water
and ethanol. Based on these preliminary preferences, some suitable
low volatility solvent candidates besides glycerol and ethylene
glycol are 1,4-butylene glycol and 1,5-pentanediol.
If it is convenient to do the deposition and aging above room
temperature, this opens up additional possibilities. One
modification would be to use a solvent that is not a liquid, but a
solid, at room temperature. This allows the potential use of many
more materials. Many of these higher melting point materials have
even lower volatility than the low volatility "room temperature
liquid solvents" (liquid solvents) have at elevated deposition and
aging temperatures. Although there is no required upper melting
point temperature, process simplicity indicates that these "room
temperature solid solvents" (solid solvents) should have melting
points less than 60 degrees C., and preferably less than 40 degrees
C. An additional desirable feature for a potential solid solvent is
that it readily solidify to an amorphous phase. This amorphous
solidification would reduce the chance of gel damage during an
accidental cooling. Additionally, this might allow the solvent to
be removed by freeze drying. An alternative approach to maintaining
the precursor temperature above the melting point of a solid
solvent is to dissolve the solid solvent in a carrier liquid. This
carrier liquid can be water, alcohol, or any other liquid typically
used in thin film aerogel/xerogel processing. The carrier liquid
could also be a compatible liquid introduced only as a carrier.
The surprisingly good behavior of glycerol and ethylene glycol give
some clues to other preferred solvents. We have identified several
solvents that may give properties slightly different than either
ethylene glycol or glycerol, yet still retain many of their
advantages. The most promising additional solvents include
1,2,4-butanetriol; 1,2,3- butanetriol; 2 methyl-propanetriol; and
2-(hydroxymethyl)-1,3-propanediol; 1-4, 1-4, butanediol; and
2-methyl-1,3-propanediol. Other potential solvents include the
polyols, either alone or in combination with ethylene glycol,
glycerol, or other solvents.
This use of a low volatility solvent allows a loosening of the
required atmospheric control during deposition, gelation, and/or
aging. This is because, that even though saturation should still
preferably be avoided, the atmospheric solvent concentration can be
lowered without excessive evaporation. This wider concentration
window can be used to allow wider variations in temperature across
the deposition chamber (especially near the wafer and any
evaporative cooling effects). An initial goal is to allow at least
a 1 degree C. temperature variation. Thus, the vapor concentration
of the low volatility solvent in the atmosphere should be such that
the condensation temperature (analogous to dew point) of the
solvent vapor is at least 1 degrees Celsius less than the
temperature of the substrate. Actually, the critical item is the
surface of the deposited sol and/or gelled sol. However, the thin
film nature of the sol keeps the temperature differences between
the sol and the substrate small. Since it is may be much easier to
measure the substrate temperature, these two temperatures will be
used interchangeably in this patent. Even though 1 degree C.
temperature uniformity may be obtainable under some conditions,
volume production will probably require at least a 3 degree C.
tolerance window, and preferably a 10 degree C. tolerance window.
However, the ultimate goal is to deposit, gel, and age in an
uncontrolled or a substantially uncontrolled atmosphere. In this
most preferred approach (a substantially uncontrolled atmosphere),
atmospheric controls during deposition, gelation, and aging are
limited to standard cleanroom temperature and humidity controls,
although the wafer and/or precursor sol may have independent
temperature controls. If this substantially uncontrolled atmosphere
allows excessive evaporation, then either passive or less
preferably, active atmospheric controls may be needed. For the
purposes of this application, passive controls are limited to the
placing the wafer in a relatively small container. This container
may be partially or fully sealed and may or may not also contain a
liquid reservoir of the solvent. However, the container will not
have exotic environmental controls for the wafer, container
atmosphere, and/or reservoir.
Another example of modification to the basic method is that, before
drying (and generally, but not necessarily, after aging), the thin
film wet gel 18 may have its pore surfaces modified with a surface
modification agent. This surface modification step replaces a
substantial number of the molecules on the pore walls with those of
another species. If a surface modifier is applied, it is generally
preferable to remove the water from the wet gel 18 before the
surface modifier is added. The water can be removed by rinsing the
wafer in pure ethanol, preferably by a low speed spin coating as
described in the solvent exchange in the first embodiment example.
This water removal is beneficial, because water will react with
many surface modification agents, such as HMDS; however, it is not
necessary. With our new glycerol-based method, surface modification
need not be performed to help prevent pore collapse, but it can be
used to impart other desirable properties to the dried gel. Some
examples of potentially desirable properties are hydrophobicity,
reduced dielectric constant, increased resistance to certain
chemicals, and improved temperature stability. Some potential
surface modifiers that may impart desirable properties include
hexamethyldisilazane (HMDS), the alkyl chlorosilanes
(trimethylchlorosilane (TMCS), dimethyldichlorosilane, etc.), the
alkylalkoxysilanes (trimethylmethoxysilane,
dimethyldimethoxysilane, etc.), phenyl compounds and fluorocarbon
compounds. One useful phenyl compound is hexaphenyldisilazane. Some
other useful phenyl compounds will typically follow the basic
formula, Ph.sub.x A.sub.y SiB.sub.(4-x-y), where, Ph is a phenolic
group, A is a reactive group such as Cl or OCH.sub.3, and B are the
remaining ligands which, if there are two, can be the same group or
different. Some examples of these phenyl surface modification
agents include compounds with 1 phenolic group such as
phenyltrichlorosilane, phenyltrifluorosilane,
phenyltrimethoxysilane, phenyltriethoxysilane,
phenylmethylchlorosilane, phenylethyldichlorosilane,
phenyldimethylethoxysilane, phenyldimethylchlorosilane,
phenyldichlorosilane, phenyl(3-chloropropyl)dichlorosilane,
phenylmethylvinylchlorosilane, phenethyldimethylchlorosilane,
phenyltrichlorosilane, phenyltrimethoxysilane,
phenyltris(trimethylsiloxy)silane, and phenylallyldichlorosilane.
Other examples of these phenyl surface modification agents include
compounds with 2 phenolic groups such as diphenyldichlorosilane,
diphenylchlorosilane, diphenylfluorosilane,
diphenylmethylchlorosilane, diphenylethylchlorosilane,
diphenyldimethoxysilane, diphenylmethoxysilane,
diphenylethoxysilane, diphenylmethylmethoxysilane,
diphenylmethylethoxysilane and diphenyidiethoxysilane. These phenyl
surface modification agents also include compounds with 3 phenolic
groups such as triphenylchlorosilane, triphenylflourosilane, and
triphenylethoxysilane. Another important phenyl compound,
1,3-diphenyltetramethyidisilazane, is an exception to this basic
formula. These lists are not exhaustive, but do convey the basic
nature of the group. The useful fluorocarbon based surface
modification agents include
(3,3,3-trifluoropropyl)trimethoxysilane),
(tridecafluoro-1,1,2,2-tetrahydrooctyl)-1 dimethylchlorsilane, and
other fluorocarbon groups that have a reactive group, such as Cl or
OCH.sub.3, that will form covalent bonds with a hydroxyl group.
The paragraph above lists some of the typical useful properties for
many conventional applications. However, there are other potential
applications for nanoporous dielectrics and aerogels that may have
different desirable properties. Examples of some other potentially
desirable properties include hydrophilicity, increased electrical
conductivity, increased dielectric breakdown voltage, increased or
decreased reactivity with certain chemicals, and increased
volatility. This list is not exhaustive. However, it shows that,
depending upon the application, many different types of properties
may be desirable. Thus, it is clear that many other materials that
will form covalent bonds with hydroxyl groups are potential surface
modifiers that may impart other potentially desirable
properties.
This invention also comprises the use of gelation catalysts, such
as ammonium hydroxide. This also includes the allowance of other
gelation catalysts in place of the ammonium hydroxide and/or for
the gelation catalyst to be added after deposition. Typically,
these alternate catalysts modify the pH of the sol. It is
preferable to use catalysts that raise the pH, although acid
catalysts can be used. Typically, acid catalysis results in longer
processing times and a denser dielectric than a base catalyzed
process. Some examples of other preferred gelation catalysts
include ammonia, the volatile amine species (low molecular weight
amines) and volatile fluorine species. When the catalyst is added
after deposition, it is preferable to add the catalyst as a vapor,
mist, or other vaporish form.
This invention allows production of nanoporous dielectrics at room
temperature and atmospheric pressure, without a separate surface
modification step. Although not required to prevent substantial
densification, this new method does not exclude the use of
supercritical drying or surface modification steps prior to drying.
To the extent that the freezing rates are fast enough to prevent
large (e.g., 50 nm) crystals, it is also compatible with freeze
drying. In general, this new method is compatible with most prior
art aerogel techniques.
Other examples of modifications involve the reaction atmosphere
and/or temperature. Also coating and gelation need not be performed
in the same chamber or even in the same atmosphere. For instance,
the substrate may have its temperature lowered to retard gelation
or elevated to speed surface modification and/or gelation. Also,
total pressure and/or temperature may be varied to further control
evaporation rates and/or gel time. Elevated temperature processing
is typically performed at no less than 40.degree. C.; however,
50.degree. C. is preferred, and 70.degree. C. is more preferred.
When working at elevated temperatures, care should be taken (e.g.,
the partial pressures in the reaction atmosphere should be high
enough) to prevent solvent boiling.
Although TEOS has been used as a representative example of a
reactant, other metal alkoxides may be used either alone or in
combination with TEOS or each other to form a silica network. These
metal alkoxides include tetramethoxysilane (TMOS),
methyltriethoxysilane (MTEOS), 1,2-Bis(trimethoxysilyl)ethane
(BTMSE), combinations thereof, and other silicon-based metal
alkoxides known in the art. A sol may also be formed from alkoxides
of other metals known in the art such as aluminum and titanium.
Some other precursor sols known in the art include particulate
metal oxides and organic precursors. Two representative particulate
metal oxides are pyrogenic (fumed) silica and colloidal silica.
Some representative organic precursors are melamine, phenol
furfural, and resorcinol. In addition to alternate reactants,
alternate solvents may also be used. Some examples of preferred
alternates for ethanol are methanol and the other higher alcohols.
Other acids may be used as a precursor sol stabilizer in place of
the nitric acid.
An additional modification is to allow and/or promote the formation
of moderate sized (15 to 150 monomers per molecule) oligomers in
the precursor sol. These larger oligomers may speed the gelation
process in the deposited sol. A sol containing large oligomers may
have a higher viscosity-than a sol with small oligomers. However,
as long as the viscosity is stable, this higher viscosity can be
compensated by methods known in the art, such as adjusting solvent
ratios and spin conditions. To help achieve this desired stable
viscosity, the oligomerization may need to be slowed or
substantially halted before deposition. Potential methods of
promoting oligomerization might include heating the precursor sol,
evaporating solvent, or adding small amounts of a gelation catalyst
such as ammonium hydroxide. Potential methods of retarding
oligomerization might include cooling the precursor sol, diluting
the sol with a solvent, or restoring the precursor sol to a pH that
minimizes condensation and gelation (Nitric acid could be used in
conjunction with the ammonium hydroxide exemplified above).
Although the present invention has been described with several
sample embodiments, various changes and modifications may be
suggested to one skilled in the art. It is intended that the
present invention encompass such changes and modifications as fall
within the scope of the appended claims.
* * * * *